Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
Message-ID: <ba78d142-6a97-99dd-9d00-465f7d6aa712@redhat.com>
Date:   Tue, 14 Dec 2021 07:18:57 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.2.0
Subject: Re: [PATCH 16/19] kvm: x86: Introduce KVM_{G|S}ET_XSAVE2 ioctl
Content-Language: en-US
To:     "Wang, Wei W" <wei.w.wang@intel.com>,
        "Zhong, Yang" <yang.zhong@intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>
Cc:     "seanjc@google.com" <seanjc@google.com>,
        "Nakajima, Jun" <jun.nakajima@intel.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        "jing2.liu@linux.intel.com" <jing2.liu@linux.intel.com>,
        "Liu, Jing2" <jing2.liu@intel.com>,
        "Zeng, Guang" <guang.zeng@intel.com>
References: <20211208000359.2853257-1-yang.zhong@intel.com>
 <20211208000359.2853257-17-yang.zhong@intel.com>
 <d16aab21-0f81-f758-a61e-5919f223be78@redhat.com>
 <26ea7039-3186-c23f-daba-d039bb8d6f48@redhat.com>
 <86d3c3a5d61649079800a2038370365b@intel.com>
 <bdda79b5-79e4-22fd-9af8-ec6e87a412ab@redhat.com>
 <3ec6019a551249d6994063e56a448625@intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <3ec6019a551249d6994063e56a448625@intel.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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
>