linux-kernel - Re: [PATCH 14/19] x86/fpu: Prepare for KVM XFD

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Message-ID: <a0bfd816-f4f9-6b98-22cc-8edd3c126041@redhat.com>
Date:   Fri, 10 Dec 2021 17:16:47 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yang Zhong <yang.zhong@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com
Cc:     seanjc@...gle.com, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, jing2.liu@...el.com
Subject: Re: [PATCH 14/19] x86/fpu: Prepare for KVM XFD_ERR handling

On 12/8/21 01:03, Yang Zhong wrote:
> 
> The guest XFD_ERR value is saved in fpu_guest::xfd_err. There is
> no need to save host XFD_ERR because it's always cleared to ZERO
> by the host #NM handler (which cannot be preempted by a vCPU
> thread to observe a non-zero value).
> 
> The lower two bits in fpu_guest::xfd_err is borrowed for special
> purposes. The state components (FP and SSE) covered by the two
> bits are not XSAVE-enabled feature, thus not XFD-enabled either.
> It's impossible to see hardware setting them in XFD_ERR:
> 
>    - XFD_ERR_GUEST_DISABLED (bit 0)
> 
>      Indicate that XFD extension is not exposed to the guest thus
>      no need to save/restore it.

Can this instead just check if xfeatures includes any dynamic-save features?

Paolo

>    - XFD_ERR_GUEST_SAVED (bit 1)
> 
>      Indicate fpu_guest::xfd_err already contains a saved value
>      thus no need for duplicated saving (e.g. when the vCPU thread
>      is preempted multiple times before re-enter the guest).