linux-kernel - Re: [PATCH 0/8] KVM: SEV-ES: fixes for string I/O emulation

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Date:   Fri, 22 Oct 2021 15:33:49 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        fwilhelm@...gle.com, oupton@...gle.com
Subject: Re: [PATCH 0/8] KVM: SEV-ES: fixes for string I/O emulation

On 22/10/21 01:49, Sean Christopherson wrote:
> On Wed, Oct 13, 2021, Paolo Bonzini wrote:
>> Patches 2 to 7 are a bunch of cleanups to emulator_pio_in and
>> emulator_pio_in_out, so that the final SEV code is a little easier
>> to reason on.  Just a little, no big promises.
> IMO, this series goes in the wrong direction and doesn't make the mess any better,
> just different.
> 
> The underlying issue is that kernel_pio() does the completely horrendous thing
> of consuming vcpu->arch.pio.  That leads to the juggling that this series tries
> to clean up, but it's essentially an impossible problem to solve because the
> approach itself is broken.

I agree on this, but I disagree that the series does not make the mess 
any better.  At the very least, the new signatures for 
__emulator_pio_in, complete_emulator_pio_in and emulator_pio_in_out are 
improvements regarding the _role_ of vcpu->arch.pio*:

- complete_emulator_pio_in clearly takes the values from vcpu->arch.pio, 
which _is_ the right thing to do for a complete_userspace_io function. 
This is not clear of emulator_pio_in before the patch

- __emulator_pio_in and emulator_pio_in_out do not take anymore the 
buffer argument, making it clear that they operate on the internal 
pio_data buffer and only complete_emulator_pio_in copies out of it. 
Which yes is horrible, but at least it is clearly visible in the code now.

I managed to clean things up quite satisfactorily with just 6 patches on 
top of these eight, so I'll post the full series as soon as I finish 
testing them.  5.15 can then include these to fix the bug at hand.

Paolo