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

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Date:   Tue, 14 Dec 2021 14:52:18 +0800
From:   "Liu, Jing2" <jing2.liu@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Yang Zhong <yang.zhong@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>, x86@...nel.org,
        kvm@...r.kernel.org, Sean Christoperson <seanjc@...gle.com>,
        Jin Nakajima <jun.nakajima@...el.com>,
        Kevin Tian <kevin.tian@...el.com>
Subject: Re: [patch 0/6] x86/fpu: Preparatory changes for guest AMX support

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