175 lines
6.5 KiB
ReStructuredText
175 lines
6.5 KiB
ReStructuredText
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Using XSTATE features in user space applications
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================================================
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The x86 architecture supports floating-point extensions which are
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enumerated via CPUID. Applications consult CPUID and use XGETBV to
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evaluate which features have been enabled by the kernel XCR0.
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Up to AVX-512 and PKRU states, these features are automatically enabled by
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the kernel if available. Features like AMX TILE_DATA (XSTATE component 18)
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are enabled by XCR0 as well, but the first use of related instruction is
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trapped by the kernel because by default the required large XSTATE buffers
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are not allocated automatically.
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The purpose for dynamic features
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--------------------------------
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Legacy userspace libraries often have hard-coded, static sizes for
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alternate signal stacks, often using MINSIGSTKSZ which is typically 2KB.
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That stack must be able to store at *least* the signal frame that the
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kernel sets up before jumping into the signal handler. That signal frame
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must include an XSAVE buffer defined by the CPU.
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However, that means that the size of signal stacks is dynamic, not static,
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because different CPUs have differently-sized XSAVE buffers. A compiled-in
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size of 2KB with existing applications is too small for new CPU features
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like AMX. Instead of universally requiring larger stack, with the dynamic
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enabling, the kernel can enforce userspace applications to have
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properly-sized altstacks.
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Using dynamically enabled XSTATE features in user space applications
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--------------------------------------------------------------------
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The kernel provides an arch_prctl(2) based mechanism for applications to
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request the usage of such features. The arch_prctl(2) options related to
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this are:
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-ARCH_GET_XCOMP_SUPP
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arch_prctl(ARCH_GET_XCOMP_SUPP, &features);
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ARCH_GET_XCOMP_SUPP stores the supported features in userspace storage of
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type uint64_t. The second argument is a pointer to that storage.
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-ARCH_GET_XCOMP_PERM
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arch_prctl(ARCH_GET_XCOMP_PERM, &features);
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ARCH_GET_XCOMP_PERM stores the features for which the userspace process
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has permission in userspace storage of type uint64_t. The second argument
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is a pointer to that storage.
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-ARCH_REQ_XCOMP_PERM
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arch_prctl(ARCH_REQ_XCOMP_PERM, feature_nr);
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ARCH_REQ_XCOMP_PERM allows to request permission for a dynamically enabled
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feature or a feature set. A feature set can be mapped to a facility, e.g.
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AMX, and can require one or more XSTATE components to be enabled.
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The feature argument is the number of the highest XSTATE component which
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is required for a facility to work.
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When requesting permission for a feature, the kernel checks the
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availability. The kernel ensures that sigaltstacks in the process's tasks
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are large enough to accommodate the resulting large signal frame. It
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enforces this both during ARCH_REQ_XCOMP_SUPP and during any subsequent
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sigaltstack(2) calls. If an installed sigaltstack is smaller than the
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resulting sigframe size, ARCH_REQ_XCOMP_SUPP results in -ENOSUPP. Also,
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sigaltstack(2) results in -ENOMEM if the requested altstack is too small
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for the permitted features.
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Permission, when granted, is valid per process. Permissions are inherited
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on fork(2) and cleared on exec(3).
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The first use of an instruction related to a dynamically enabled feature is
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trapped by the kernel. The trap handler checks whether the process has
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permission to use the feature. If the process has no permission then the
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kernel sends SIGILL to the application. If the process has permission then
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the handler allocates a larger xstate buffer for the task so the large
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state can be context switched. In the unlikely cases that the allocation
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fails, the kernel sends SIGSEGV.
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AMX TILE_DATA enabling example
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Below is the example of how userspace applications enable
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TILE_DATA dynamically:
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1. The application first needs to query the kernel for AMX
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support::
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#include <asm/prctl.h>
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#include <sys/syscall.h>
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#include <stdio.h>
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#include <unistd.h>
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#ifndef ARCH_GET_XCOMP_SUPP
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#define ARCH_GET_XCOMP_SUPP 0x1021
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#endif
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#ifndef ARCH_XCOMP_TILECFG
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#define ARCH_XCOMP_TILECFG 17
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#endif
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#ifndef ARCH_XCOMP_TILEDATA
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#define ARCH_XCOMP_TILEDATA 18
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#endif
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#define MASK_XCOMP_TILE ((1 << ARCH_XCOMP_TILECFG) | \
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(1 << ARCH_XCOMP_TILEDATA))
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unsigned long features;
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long rc;
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...
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rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_SUPP, &features);
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if (!rc && (features & MASK_XCOMP_TILE) == MASK_XCOMP_TILE)
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printf("AMX is available.\n");
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2. After that, determining support for AMX, an application must
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explicitly ask permission to use it::
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#ifndef ARCH_REQ_XCOMP_PERM
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#define ARCH_REQ_XCOMP_PERM 0x1023
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#endif
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...
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rc = syscall(SYS_arch_prctl, ARCH_REQ_XCOMP_PERM, ARCH_XCOMP_TILEDATA);
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if (!rc)
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printf("AMX is ready for use.\n");
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Note this example does not include the sigaltstack preparation.
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Dynamic features in signal frames
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---------------------------------
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Dynamcally enabled features are not written to the signal frame upon signal
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entry if the feature is in its initial configuration. This differs from
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non-dynamic features which are always written regardless of their
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configuration. Signal handlers can examine the XSAVE buffer's XSTATE_BV
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field to determine if a features was written.
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Dynamic features for virtual machines
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-------------------------------------
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The permission for the guest state component needs to be managed separately
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from the host, as they are exclusive to each other. A coupled of options
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are extended to control the guest permission:
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-ARCH_GET_XCOMP_GUEST_PERM
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arch_prctl(ARCH_GET_XCOMP_GUEST_PERM, &features);
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ARCH_GET_XCOMP_GUEST_PERM is a variant of ARCH_GET_XCOMP_PERM. So it
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provides the same semantics and functionality but for the guest
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components.
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-ARCH_REQ_XCOMP_GUEST_PERM
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arch_prctl(ARCH_REQ_XCOMP_GUEST_PERM, feature_nr);
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ARCH_REQ_XCOMP_GUEST_PERM is a variant of ARCH_REQ_XCOMP_PERM. It has the
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same semantics for the guest permission. While providing a similar
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functionality, this comes with a constraint. Permission is frozen when the
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first VCPU is created. Any attempt to change permission after that point
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is going to be rejected. So, the permission has to be requested before the
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first VCPU creation.
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Note that some VMMs may have already established a set of supported state
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components. These options are not presumed to support any particular VMM.
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