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Re: [PATCH v22 8/9] target-arm: kvm64: handle SIGBUS signal from kernel

From: Peter Maydell
Subject: Re: [PATCH v22 8/9] target-arm: kvm64: handle SIGBUS signal from kernel or KVM
Date: Thu, 16 Jan 2020 16:40:01 +0000

On Thu, 16 Jan 2020 at 16:28, Peter Maydell <address@hidden> wrote:
> This function (kvm_arch_on_sigbus_vcpu()) will be called in two contexts:
> (1) in the vcpu thread:
>   * the real SIGBUS handler sigbus_handler() sets a flag and arranges
>     for an immediate vcpu exit
>   * the vcpu thread reads the flag on exit from KVM_RUN and
>     calls kvm_arch_on_sigbus_vcpu() directly
>   * the error could be MCEERR_AR or MCEERR_AO
> (2) MCE errors on other threads:
>   * here SIGBUS is blocked, so MCEERR_AR (action-required)
>     errors will cause the kernel to just kill the QEMU process
>   * MCEERR_AO errors will be handled via the iothread's use
>     of signalfd(), so kvm_on_sigbus() will get called from
>     the main thread, and it will call kvm_arch_on_sigbus_vcpu()
>   * in this case the passed in CPUState will (arbitrarily) be that
>     for the first vCPU
> For MCEERR_AR, the code here looks correct -- we know this is
> only going to happen for the relevant vCPU so we can go ahead
> and do the "record it in the ACPI table and tell the guest" bit.
> But shouldn't we be doing something with the MCEERR_AO too?
> That of course will be trickier because we're not necessarily
> in the vcpu thread, but it would be nice to let the guest
> know about it.

An IRC discussion with Paolo came to the conclusion that
the nicest approach here would be for kvm_on_sigbus() to
use run_on_cpu() to call the whole of kvm_arch_on_sigbus_vcpu()
in the vcpu thread for the cpu it gets passed. Then the code
here would not have to worry about the "not on the right thread"
case. This would be a refactoring of the x86 code, which currently
does the run_on_cpu inside its implementation, in

-- PMM

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