linux-kernel - Re: [PATCH 4/8] KVM: SVM: move guest vmsave/vmload to assembly

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Date:   Mon, 7 Nov 2022 16:40:23 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        nathan@...nel.org, thomas.lendacky@....com,
        andrew.cooper3@...rix.com, jmattson@...gle.com, seanjc@...gle.com,
        stable@...r.kernel.org
Subject: Re: [PATCH 4/8] KVM: SVM: move guest vmsave/vmload to assembly

On 11/7/22 16:23, Peter Zijlstra wrote:
>>   
>> +3:	vmrun %_ASM_AX
>> +4:
>> +	cli
>>   
>> +	/* Pop @svm to RAX while it's the only available register. */
>>   	pop %_ASM_AX
>>   
>>   	/* Save all guest registers.  */
> So Andrew noted that once the vmload has executed any exception taken
> (say at 3) will crash and burn because %gs is scribbled.
> 
> Might be good to make a record of this in the code so it can be cleaned
> up some day.
> 

Yeah, it won't happen because clgi/stgi blocks setting kvm_rebooting so 
I thought of killing the three exception fixups after the first.  In the 
end I kept them for simplicity and to keep the normal/SEV-ES versions as 
similar as possible.

Paolo