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

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Message-ID: <ba78d142-6a97-99dd-9d00-465f7d6aa712@redhat.com>
Date:   Tue, 14 Dec 2021 07:18:57 +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 12/14/21 07:06, Wang, Wei W wrote:
> 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.

You can use KVM_ENABLE_CAP(KVM_CAP_XSAVE2) for that, yes.  In that case 
you don't need KVM_GET_XSAVE2.

Paolo

> - 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
>