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

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Date:   Tue, 14 Dec 2021 06:06:53 +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 Monday, December 13, 2021 5:24 PM, Paolo Bonzini wrote:
> There is no need for struct kvm_xsave2, because there is no need for a "size"
> argument.
> 
> - KVM_GET_XSAVE2 *is* needed, and it can expect a buffer as big as the return
> value of KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2)

Why would KVM_GET_XSAVE2 still be needed in this case?

I'm thinking it would also be possible to reuse KVM_GET_XSAVE:

- If userspace calls to KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2),
 then KVM knows that the userspace is a new version and it works with larger xsave buffer using the "size" that it returns via KVM_CAP_XSAVE2.
 So we can add a flag "kvm->xsave2_enabled", which gets set upon userspace checks KVM_CAP_XSAVE2.

- On KVM_GET_XSAVE, if "kvm->xsave2_enabled" is set,
 then KVM allocates buffer to load xstates and copies the loaded xstates data to the userspace buffer
 using the "size" that was returned to userspace on KVM_CAP_XSAVE2.
 If "kvm->xsave2_enabled" isn't set, using the legacy "4KB" size.

Thanks,
Wei