Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
Message-ID: <a0bfd816-f4f9-6b98-22cc-8edd3c126041@redhat.com>
Date:   Fri, 10 Dec 2021 17:16:47 +0100
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Subject: Re: [PATCH 14/19] x86/fpu: Prepare for KVM XFD_ERR handling
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To:     Yang Zhong <yang.zhong@intel.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com
Cc:     seanjc@google.com, jun.nakajima@intel.com, kevin.tian@intel.com,
        jing2.liu@linux.intel.com, jing2.liu@intel.com
References: <20211208000359.2853257-1-yang.zhong@intel.com>
 <20211208000359.2853257-15-yang.zhong@intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <20211208000359.2853257-15-yang.zhong@intel.com>
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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).