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Using XSTATE features in user space applications
================================================
The x86 architecture supports floating-point extensions which are
enumerated via CPUID. Applications consult CPUID and use XGETBV to
evaluate which features have been enabled by the kernel XCR0.
Up to AVX-512 and PKRU states, these features are automatically enabled by
the kernel if available. Features like AMX TILE_DATA (XSTATE component 18)
are enabled by XCR0 as well, but the first use of related instruction is
trapped by the kernel because by default the required large XSTATE buffers
are not allocated automatically.
Using dynamically enabled XSTATE features in user space applications
--------------------------------------------------------------------
The kernel provides an arch_prctl(2) based mechanism for applications to
request the usage of such features. The arch_prctl(2) options related to
this are:
-ARCH_GET_XCOMP_SUPP
arch_prctl(ARCH_GET_XCOMP_SUPP, &features);
ARCH_GET_XCOMP_SUPP stores the supported features in userspace storage of
type uint64_t. The second argument is a pointer to that storage.
-ARCH_GET_XCOMP_PERM
arch_prctl(ARCH_GET_XCOMP_PERM, &features);
ARCH_GET_XCOMP_PERM stores the features for which the userspace process
has permission in userspace storage of type uint64_t. The second argument
is a pointer to that storage.
-ARCH_REQ_XCOMP_PERM
arch_prctl(ARCH_REQ_XCOMP_PERM, feature_nr);
ARCH_REQ_XCOMP_PERM allows to request permission for a dynamically enabled
feature or a feature set. A feature set can be mapped to a facility, e.g.
AMX, and can require one or more XSTATE components to be enabled.
The feature argument is the number of the highest XSTATE component which
is required for a facility to work.
When requesting permission for a feature, the kernel checks the
availability. The kernel ensures that sigaltstacks in the process's tasks
are large enough to accommodate the resulting large signal frame. It
enforces this both during ARCH_REQ_XCOMP_SUPP and during any subsequent
sigaltstack(2) calls. If an installed sigaltstack is smaller than the
resulting sigframe size, ARCH_REQ_XCOMP_SUPP results in -ENOSUPP. Also,
sigaltstack(2) results in -ENOMEM if the requested altstack is too small
for the permitted features.
Permission, when granted, is valid per process. Permissions are inherited
on fork(2) and cleared on exec(3).
The first use of an instruction related to a dynamically enabled feature is
trapped by the kernel. The trap handler checks whether the process has
permission to use the feature. If the process has no permission then the
kernel sends SIGILL to the application. If the process has permission then
the handler allocates a larger xstate buffer for the task so the large
state can be context switched. In the unlikely cases that the allocation
fails, the kernel sends SIGSEGV.
Dynamic features in signal frames
---------------------------------
Dynamcally enabled features are not written to the signal frame upon signal
entry if the feature is in its initial configuration. This differs from
non-dynamic features which are always written regardless of their
configuration. Signal handlers can examine the XSAVE buffer's XSTATE_BV
field to determine if a features was written.
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