linux-kernel - Re: [PATCH v2 18/23] kvm: x86: Get/set expanded xstate buffer

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Date:   Tue, 21 Dec 2021 09:45:24 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Wang, Wei W" <wei.w.wang@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Cc:     "seanjc@...gle.com" <seanjc@...gle.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "jing2.liu@...ux.intel.com" <jing2.liu@...ux.intel.com>,
        "Zeng, Guang" <guang.zeng@...el.com>,
        "Zhong, Yang" <yang.zhong@...el.com>
Subject: Re: [PATCH v2 18/23] kvm: x86: Get/set expanded xstate buffer

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