linux-kernel - RE: [PATCH v3 19/22] kvm: x86: Get/set expanded xstate buffer

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Date:   Wed, 29 Dec 2021 06:36:43 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     "Wang, Wei W" <wei.w.wang@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Liu, Jing2" <jing2.liu@...el.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...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>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Nakajima, Jun" <jun.nakajima@...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 v3 19/22] kvm: x86: Get/set expanded xstate buffer

> From: Wang, Wei W <wei.w.wang@...el.com>
> Sent: Wednesday, December 29, 2021 10:58 AM
> 
> > > Reuse KVM_SET_XSAVE for both old/new formats by reimplementing it to
> > > do properly-sized memdup_user() based on the guest fpu container.
> >
> > I'm confused, the first sentence says KVM_SET_XSAVE isn't suitable, the
> > second says it can be reused with minimal effort.
> 
> Probably "doesn't support" sounds better than "isn't suitable" above. But
> plan to reword a bit:
> 
> With KVM_CAP_XSAVE, userspace uses a hardcoded 4KB buffer to get/set
> xstate data from/to
> KVM. This doesn't work when dynamic features (e.g. AMX) are used by the
> guest, as KVM uses
> a full expanded xstate buffer for the guest fpu emulation, which is larger
> than 4KB.
> 
> Add KVM_CAP_XSAVE2, and userspace gets the required xstate buffer size
> from KVM via
> KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2). KVM_SET_XSAVE is extended
> with the support to
> work with larger xstate data size passed from userspace. KVM_GET_XSAVE2
> is preferred to
> extending KVM_GET_XSAVE to work with large buffer size for backward-
> compatible considerations.
> (Link: https://lkml.org/lkml/2021/12/15/510)
> 
> Also, update the api doc with the new KVM_GET_XSAVE2 ioctl.
> 

Revised to:

--

With KVM_CAP_XSAVE, userspace uses a hardcoded 4KB buffer to get/set
xstate data from/to KVM. This doesn't work when dynamic xfeatures
(e.g. AMX) are exposed to the guest as they require a larger buffer
size.

Introduce a new capability (KVM_CAP_XSAVE2). Userspace VMM gets the
required xstate buffer size via KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2).
KVM_SET_XSAVE is extended to work with both legacy and new capabilities
by doing properly-sized memdup_user() based on the guest fpu container.
KVM_GET_XSAVE is kept for backward-compatible reason. Instead,
KVM_GET_XSAVE2 is introduced under KVM_CAP_XSAVE2 as the preferred
interface for getting xstate buffer (4KB or larger size) from KVM.
(Link: https://lkml.org/lkml/2021/12/15/510)

Also, update the api doc with the new KVM_GET_XSAVE2 ioctl

--

Thanks
Kevin