Messages in this thread | | | Date | Fri, 29 Sep 2023 10:23:54 +0100 | From | Marc Zyngier <> | Subject | Re: [PATCH v2 1/2] KVM: arm64: Add handler for MOPS exceptions |
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On Wed, 27 Sep 2023 09:28:20 +0100, Oliver Upton <oliver.upton@linux.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.
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