Re: [patch 0/6] x86/fpu: Preparatory changes for guest AMX support

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Hi Thomas,

On 12/14/2021 10:50 AM, Thomas Gleixner wrote:
Folks,

this is a follow up to the initial sketch of patches which got picked up by
Jing and have been posted in combination with the KVM parts:

    https://lore.kernel.org/r/20211208000359.2853257-1-yang.zhong@xxxxxxxxx

This update is only touching the x86/fpu code and not changing anything on
the KVM side.

     BIG FAT WARNING: This is compile tested only!

In course of the dicsussion of the above patchset it turned out that there
are a few conceptual issues vs. hardware and software state and also
vs. guest restore.

This series addresses this with the following changes vs. the original
approach:

   1) fpstate reallocation is now independent of fpu_swap_kvm_fpstate()

      It is triggered directly via XSETBV and XFD MSR write emulation which
      are used both for runtime and restore purposes.

      For this it provides two wrappers around a common update function, one
      for XCR0 and one for XFD.

      Both check the validity of the arguments and the correct sizing of the
      guest FPU fpstate. If the size is not sufficient, fpstate is
      reallocated.

      The functions can fail.

   2) XFD synchronization

      KVM must neither touch the XFD MSR nor the fpstate->xfd software state
      in order to guarantee state consistency.

      In the MSR write emulation case the XFD specific update handler has to
      be invoked. See #1

      If MSR write emulation is disabled because the buffer size is
      sufficient for all use cases, i.e.:

      		guest_fpu::xfeatures == guest_fpu::perm

The buffer size can be sufficient once one of the features is requested since kernel fpu realloc full size (permitted). And I think we don't want to disable
interception until all the features are detected e.g., one by one.

Thus it can be guest_fpu::xfeatures != guest_fpu::perm.


Thanks,
Jing




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