Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
Message-ID: <3ffa47eb-3555-5925-1c55-f89a07ceb4bc@redhat.com>
Date:   Mon, 20 Dec 2021 10:04:14 +0100
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Subject: Re: [PATCH v2 18/23] kvm: x86: Get/set expanded xstate buffer
Content-Language: en-US
To:     Jing Liu <jing2.liu@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, guang.zeng@intel.com,
        wei.w.wang@intel.com, yang.zhong@intel.com
References: <20211217153003.1719189-1-jing2.liu@intel.com>
 <20211217153003.1719189-19-jing2.liu@intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <20211217153003.1719189-19-jing2.liu@intel.com>
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On 12/17/21 16:29, Jing Liu wrote:
> +/* for KVM_CAP_XSAVE and KVM_CAP_XSAVE2 */
>   struct kvm_xsave {
> +	/*
> +	 * KVM_GET_XSAVE only uses the first 4096 bytes.
> +	 *
> +	 * KVM_GET_XSAVE2 must have the size match what is returned by
> +	 * KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2).
> +	 *
> +	 * KVM_SET_XSAVE uses the extra field if guest_fpu::fpstate::size
> +	 * exceeds 4096 bytes.

KVM_GET_XSAVE2 and KVM_SET_XSAVE respectively write and read as many 
bytes as are returned by KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2), when 
invoked on the vm file descriptor.  Currently, 
KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will only return a value that is 
greater than 4096 bytes if any dynamic features have been enabled with 
``arch_prctl()``; this however may change in the future.

The offsets of the state save areas in struct kvm_xsave follow the 
contents of CPUID leaf 0xD on the host.

Paolo