Message-ID: <34ad15d6-d525-1fe0-8aa7-92a52a19861c@redhat.com>
Date:   Tue, 21 Dec 2021 10:30:47 +0100
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Subject: Re: [PATCH v2 18/23] kvm: x86: Get/set expanded xstate buffer
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To:     "Wang, Wei W" <wei.w.wang@intel.com>,
        "Liu, Jing2" <jing2.liu@intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>
Cc:     "seanjc@google.com" <seanjc@google.com>,
        "Nakajima, Jun" <jun.nakajima@intel.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        "jing2.liu@linux.intel.com" <jing2.liu@linux.intel.com>,
        "Zeng, Guang" <guang.zeng@intel.com>,
        "Zhong, Yang" <yang.zhong@intel.com>
References: <20211217153003.1719189-1-jing2.liu@intel.com>
 <20211217153003.1719189-19-jing2.liu@intel.com>
 <3ffa47eb-3555-5925-1c55-f89a07ceb4bc@redhat.com>
 <e0fd378de64f44fd8becfe67b02cb635@intel.com>
 <219a751e-ac2d-9ce1-9db7-7d5b1edd6bdd@redhat.com>
 <c06fdc4f3b4d4346ae80801a6c3a6ff2@intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <c06fdc4f3b4d4346ae80801a6c3a6ff2@intel.com>
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On 12/21/21 10:06, Wang, Wei W wrote:
>> (I'm not sure if the first sentence is true in the code, but if not it is a bug that
>> has to be fixed :)).
> For the implementation, KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) always return kvm->vcpus[0]->arch.guest_fpu.uabi_size.
> Do you want to change it to below?
> 
> If (kvm->vcpus[0]->arch.guest_fpu.uabi_size < 4096)
> 	return 0;

return 4096;

since the minimum size of struct kvm_xsave2 (with no extra) is 4096.

Paolo

> else
> 	return kvm->vcpus[0]->arch.guest_fpu.uabi_size;
> 
> If the size is less than 4096 (e.g. no dynamic xfeatures enabled),
> userspace should use the old KVM_GET_XSAVE (instead of KVM_GET_XSAVE2)?
> (KVM_GET_XSAVE2 supports to work with size less than 4096, so I think this isn't necessary)