linux-kernel - Re: [PATCH 2/7] KVM: SVM: Use host_vmcb

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Message-ID: <20110714131036.GC24072@8bytes.org>
Date:	Thu, 14 Jul 2011 15:10:37 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Joerg Roedel <joerg.roedel@....com>,
	Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/7] KVM: SVM: Use host_vmcb_pa for vmload and vmsave

On Thu, Jul 14, 2011 at 02:29:36PM +0300, Avi Kivity wrote:
> On 07/13/2011 06:32 PM, Joerg Roedel wrote:
>> This saves copying over the vmload/vmsave switched part from
>> the host to the guest vmcb later.
>>
>> Signed-off-by: Joerg Roedel<joerg.roedel@....com>
>> ---
>>   arch/x86/kvm/svm.c |    7 ++++++-
>>   1 files changed, 6 insertions(+), 1 deletions(-)
>>
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index 3d5990f..dc703ac 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -3704,9 +3704,13 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>>
>>   		/* Enter guest mode */
>>   		"push %%"R"ax \n\t"
>> -		"mov %c[vmcb](%[svm]), %%"R"ax \n\t"
>> +		"mov %c[host_vmcb](%[svm]), %%"R"ax \n\t"
>>   		__ex(SVM_VMLOAD) "\n\t"
>> +		"mov (%%"R"sp), %%"R"ax\n\t"
>> +		"mov %c[vmcb](%[svm]), %%"R"ax \n\t"
>>   		__ex(SVM_VMRUN) "\n\t"
>> +		"mov (%%"R"sp), %%"R"ax\n\t"
>> +		"mov %c[host_vmcb](%[svm]), %%"R"ax \n\t"
>>   		__ex(SVM_VMSAVE) "\n\t"
>>   		"pop %%"R"ax \n\t"
>>
>
> Okay, so the plan is to split L2 state between ->vmcb and ->host_vmcb?

Yes, otherwise we need to copy the vmload/vmsave switched state back and
forth between both VMCBs which is a waste of cycles.

> In that case my suggestion for patch 1 doesn't apply.  But the name  
> still is confusing.  If we don't find a better one, I want a fat comment  
> explaining how state is split.

Hmm, how about naming them l1_vmcb and l2_vmcb? The comment explaining
why vmload/vmsave always happens on l1_vmcb is needed anyway then.

> (would be good to have documentation for the overall strategy of nsvm,  
> like we have for nvmx and nmmu).

There is not much to document about future plans for nested-svm. At the
moment I try to add emulation code for new SVM features when there is
some time left. Live migration support is also on the list.

The long-term plan is certainly to merge code with nested-vmx where
possible and move logic into generic KVM code. The first item that comes
to mind here is to create a single place where a vmexit is emulated and
let all other place which do that today just signal that it is required.

	Joerg

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