linux-kernel - Re: [PATCH 16/19] kvm: x86: Introduce KVM_{G|S}ET

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Date:   Thu, 16 Dec 2021 11:28:02 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Wang, Wei W" <wei.w.wang@...el.com>,
        "Zhong, Yang" <yang.zhong@...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>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Zeng, Guang" <guang.zeng@...el.com>
Subject: Re: [PATCH 16/19] kvm: x86: Introduce KVM_{G|S}ET_XSAVE2 ioctl

On 12/16/21 09:25, Wang, Wei W wrote:
>> It's still easier to return the full size of the buffer from 
>> KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2).  It makes the userspace code
>> a bit easier.
> 
> OK. For the "full size" returned to userspace, would you prefer to
> directly use the value retrieved from guest CPUID(0xd), or get it
> from guest_fpu (i.e. fpstate->user_size)? (retrieved from CPUID will
> be the max size and should work fine as well)

It is okay to reflect only the bits that were enabled in prctl, but 
please document it in api.rst as well.

Paolo