trapped by the kernel because by default the required large XSTATE buffers
are not allocated automatically.
+The purpose for dynamic features
+--------------------------------
+
+Legacy userspace libraries often have hard-coded, static sizes for
+alternate signal stacks, often using MINSIGSTKSZ which is typically 2KB.
+That stack must be able to store at *least* the signal frame that the
+kernel sets up before jumping into the signal handler. That signal frame
+must include an XSAVE buffer defined by the CPU.
+
+However, that means that the size of signal stacks is dynamic, not static,
+because different CPUs have differently-sized XSAVE buffers. A compiled-in
+size of 2KB with existing applications is too small for new CPU features
+like AMX. Instead of universally requiring larger stack, with the dynamic
+enabling, the kernel can enforce userspace applications to have
+properly-sized altstacks.
+
Using dynamically enabled XSTATE features in user space applications
--------------------------------------------------------------------
state can be context switched. In the unlikely cases that the allocation
fails, the kernel sends SIGSEGV.
+AMX TILE_DATA enabling example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Below is the example of how userspace applications enable
+TILE_DATA dynamically:
+
+ 1. The application first needs to query the kernel for AMX
+ support::
+
+ #include <asm/prctl.h>
+ #include <sys/syscall.h>
+ #include <stdio.h>
+ #include <unistd.h>
+
+ #ifndef ARCH_GET_XCOMP_SUPP
+ #define ARCH_GET_XCOMP_SUPP 0x1021
+ #endif
+
+ #ifndef ARCH_XCOMP_TILECFG
+ #define ARCH_XCOMP_TILECFG 17
+ #endif
+
+ #ifndef ARCH_XCOMP_TILEDATA
+ #define ARCH_XCOMP_TILEDATA 18
+ #endif
+
+ #define MASK_XCOMP_TILE ((1 << ARCH_XCOMP_TILECFG) | \
+ (1 << ARCH_XCOMP_TILEDATA))
+
+ unsigned long features;
+ long rc;
+
+ ...
+
+ rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_SUPP, &features);
+
+ if (!rc && (features & MASK_XCOMP_TILE) == MASK_XCOMP_TILE)
+ printf("AMX is available.\n");
+
+ 2. After that, determining support for AMX, an application must
+ explicitly ask permission to use it::
+
+ #ifndef ARCH_REQ_XCOMP_PERM
+ #define ARCH_REQ_XCOMP_PERM 0x1023
+ #endif
+
+ ...
+
+ rc = syscall(SYS_arch_prctl, ARCH_REQ_XCOMP_PERM, ARCH_XCOMP_TILEDATA);
+
+ if (!rc)
+ printf("AMX is ready for use.\n");
+
+Note this example does not include the sigaltstack preparation.
+
Dynamic features in signal frames
---------------------------------
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.
+
+Dynamic features for virtual machines
+-------------------------------------
+
+The permission for the guest state component needs to be managed separately
+from the host, as they are exclusive to each other. A coupled of options
+are extended to control the guest permission:
+
+-ARCH_GET_XCOMP_GUEST_PERM
+
+ arch_prctl(ARCH_GET_XCOMP_GUEST_PERM, &features);
+
+ ARCH_GET_XCOMP_GUEST_PERM is a variant of ARCH_GET_XCOMP_PERM. So it
+ provides the same semantics and functionality but for the guest
+ components.
+
+-ARCH_REQ_XCOMP_GUEST_PERM
+
+ arch_prctl(ARCH_REQ_XCOMP_GUEST_PERM, feature_nr);
+
+ ARCH_REQ_XCOMP_GUEST_PERM is a variant of ARCH_REQ_XCOMP_PERM. It has the
+ same semantics for the guest permission. While providing a similar
+ functionality, this comes with a constraint. Permission is frozen when the
+ first VCPU is created. Any attempt to change permission after that point
+ is going to be rejected. So, the permission has to be requested before the
+ first VCPU creation.
+
+Note that some VMMs may have already established a set of supported state
+components. These options are not presumed to support any particular VMM.