Message-ID: <219a751e-ac2d-9ce1-9db7-7d5b1edd6bdd@redhat.com>
Date:   Tue, 21 Dec 2021 09:45:24 +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>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <e0fd378de64f44fd8becfe67b02cb635@intel.com>
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On 12/21/21 03:45, Wang, Wei W wrote:
>> 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.
> Would this make people think that KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) doesn’t
> return the value (i.e. return 0) if it is smaller than 4096?
> (i.e. KVM_GET_XSAVE2 doesn't work with size < 4096, which isn’t true)
> 
> I plan to just reword a bit:
> Currently, KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will only return a size value,
> and the value is greater than 4096 bytes if any dynamic features have been enabled with
> ``arch_prctl()``. More types of values could be returned in the future.

Next refinement:

The size value returned by KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will 
always be at least 4096.  Currently, it is only greater than 4096 if a 
dynamic feature has been enabled with ``arch_prctl()``, but this may 
change in the future.

(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 :)).

Paolo