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SubjectRe: [patch 0/6] x86/fpu: Preparatory changes for guest AMX support
From
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@intel.com
>
> 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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