75 lines
3.2 KiB
ReStructuredText
75 lines
3.2 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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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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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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