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

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Date:   Mon, 13 Dec 2021 08:23:16 +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 Saturday, December 11, 2021 6:13 AM, Paolo Bonzini wrote:
> 
> By the way, I think KVM_SET_XSAVE2 is not needed.  Instead:
> 
> - KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) should return the size of the
> buffer that is passed to KVM_GET_XSAVE2
> 
> - KVM_GET_XSAVE2 should fill in the buffer expecting that its size is
> whatever KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) passes
> 
> - KVM_SET_XSAVE can just expect a buffer that is bigger than 4k if the
> save states recorded in the header point to offsets larger than 4k.

I think one issue is that KVM_SET_XSAVE works with "struct kvm_xsave" (hardcoded 4KB buffer),
including kvm_vcpu_ioctl_x86_set_xsave. The states obtained via KVM_GET_XSAVE2 will be made
using "struct kvm_xsave2".

Did you mean that we could add a new code path under KVM_SET_XSAVE to make it work with
the new "struct kvm_xsave2"?
e.g.:

(xsave2_enabled below is set when userspace calls to get KVM_CAP_XSAVE2)
if (kvm->xsave2_enabled) {
	new implementation using "struct kvm_xsave2"
} else {
	current implementation using "struct kvm_xsave"
}
(this seems like a new implementation which might deserve a new ioctl)

Thanks,
Wei