linux-kernel - Re: [PATCH v2 1/2] KVM: arm64: Add handler for MOPS exceptions

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Date:   Fri, 29 Sep 2023 10:23:54 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     Kristina Martsenko <kristina.martsenko@....com>,
        kvmarm@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Colton Lewis <coltonlewis@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] KVM: arm64: Add handler for MOPS exceptions

On Wed, 27 Sep 2023 09:28:20 +0100,
Oliver Upton <oliver.upton@...ux.dev> wrote:
> 
> On Mon, Sep 25, 2023 at 04:16:06PM +0100, Kristina Martsenko wrote:
> 
> [...]
> 
> > > What is the rationale for advancing the state machine? Shouldn't we
> > > instead return to the guest and immediately get the SS exception,
> > > which in turn gets reported to userspace? Is it because we rollback
> > > the PC to a previous instruction?
> > 
> > Yes, because we rollback the PC to the prologue instruction. We advance the
> > state machine so that the SS exception is taken immediately upon returning to
> > the guest at the prologue instruction. If we didn't advance it then we would
> > return to the guest, execute the prologue instruction, and then take the SS
> > exception on the middle instruction. Which would be surprising as userspace
> > would see the middle and epilogue instructions executed multiple times but not
> > the prologue.
> 
> I agree with Kristina that taking the SS exception on the prologue is
> likely the best course of action. Especially since it matches the
> behavior of single-stepping an EL0 MOPS sequence with an intervening CPU
> migration.
> 
> This behavior might throw an EL1 that single-steps itself for a loop,
> but I think it is impossible for a hypervisor to hide the consequences
> of vCPU migration with MOPS in the first place.
> 
> Marc, I'm guessing you were most concerned about the former case where
> the VMM was debugging the guest. Is there something you're concerned
> about I missed?

My concern is not only the VMM, but any userspace that perform
single-stepping. Imagine the debugger tracks PC by itself, and simply
increments it by 4 on a non-branch, non-fault instruction.

Move the vcpu or the userspace around, rewind PC, and now the debugger
is out of whack with what is executing. While I agree that there is
not much a hypervisor can do about that, I'm a bit worried that we are
going to break existing SW with this.

Now the obvious solution is "don't do that"...

	M.

-- 
Without deviation from the norm, progress is not possible.