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

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Date:   Thu, 16 Dec 2021 08:25:54 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.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 Wednesday, December 15, 2021 9:43 PM, Paolo Bonzini 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)

> 
> I'm also thinking that I prefer KVM_GET_XSAVE2 to
> KVM_ENABLE_CAP(KVM_CAP_XSAVE2), after all.  Since it would be a
> backwards-incompatible change to an _old_ ioctl (KVM_GET_XSAVE), I prefer
> to limit the ways that userspace can shoot itself in the foot.

OK, we will use KVM_GET_XSAVE2.

Thanks,
Wei