Re: [Qemu-devel] [Qemu-arm] [PATCH RESEND v15 10/10] target-arm: kvm64: handle SIGBUS signal from kernel or KVM

[Peter Maydell]

On Mon, 26 Nov 2018 at 17:18, gengdongjiu <address@hidden> wrote:
>
> Hi Peter,
>    Thanks for the comments.
>
> >
> > Yes, but if you look at the code which calls that, it goes on to do:
> >         env->mcg_cap &= mcg_cap | MCG_CAP_BANKS_MASK;
> >
> > which means that if the host kernel does not support this feature then we 
> > will clear those bits in the env->mcg_cap field, so we do not
> > advertise it to the guest. But we might be not advertising it to the guest 
> > at all, if env->mcg_cap was 0 before this code was called. That
> > happens if we are presenting the guest with a guest CPU type which does not 
> > have the feature.
>
> So you mean "env->mcg_cap" uses a bit to indicate that whether guest CPU 
> support error recovery, right? If so, how we know whether guest CPU have this 
> feature?
> Should we initialized MCG_SER_P bit of env->mcg_cap to 1 or 0 before 
> initializing the vcpu?  Please together see the reply in the [2], thanks.

This is an x86-specific field. We need to identify the Arm
equivalent.

> > The question is not "does the host CPU / KVM support error reporting". It 
> > is "does the *guest* CPU / system support error reporting".
> > These are distinct questions which may not have the same answer.
>
> [2]:
> But QEMU should not know whether *guest* CPU / system support error reporting.

It should and it must. Otherwise we will deliver
an exception to a guest that cannot handle it.

> From my understanding, in the X86 platform, if host CPU / KVM support error 
> reporting,
> then it will think guest CPU supports error reporting, and set the
> MCG_SER_P bit of env->mcg_cap to 1

No, this is not correct. If you look at the code quoted above.
it ANDs in the mcg_cap value. This code is run after the
CPU is initially created with a default env->mcg_cap which
depends on what CPU model is being exposed to the guest.
So we only set env->mcg_cap to 1 if the guest vCPU model
supports error reporting AND the host KVM does.

thanks
-- PMM