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

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Message-ID: <bdda79b5-79e4-22fd-9af8-ec6e87a412ab@redhat.com>
Date:   Mon, 13 Dec 2021 10:24:08 +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 12/13/21 09:23, Wang, Wei W wrote:
> 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"?

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)

- but KVM_SET_XSAVE2 is not needed, because KVM_SET_XSAVE can use 
copy_from_user to read the XSTATE_BV, use it to deduce the size of the 
buffer, and use copy_from_user to read the full size of the buffer.

For this to work you can redefine struct kvm_xsave to

	struct kvm_xsave {
		__u32 region[1024];

		/*
		 * KVM_GET_XSAVE only uses 4096 bytes and only returns
		 * user save states up to save state 17 (TILECFG).
		 *
		 * For KVM_GET_XSAVE2, the total size of region + extra
		 * must be the size that is communicated by
		 * KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2).
		 *
		 * KVM_SET_XSAVE uses the extra field if the struct was
		 * returned by KVM_GET_XSAVE2.
		 */
		__u32 extra[];
	}

Paolo