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

[Peter Maydell]

On Fri, 23 Nov 2018 at 14:31, gengdongjiu <address@hidden> wrote:
>
> Hi Peter,
>   Thanks for the comments and mail.
>
> >
> > On 22 November 2018 at 10:28, Peter Maydell <address@hidden> wrote:
> > > On 22 November 2018 at 03:05, gengdongjiu <address@hidden> wrote:
> > >>> >
> > >>> Shouldn't there be something in here to say "only report this error to 
> > >>> the guest if we are actually reporting RAS errors to the guest" ?
> > >>
> > >> Yes, We can say something that such as "report this error to the guest", 
> > >> because this error is indeed triggered by guest, which is guest
> > error.
> > >
> > > I'm afraid I don't really understand what you mean. Could you try
> > > rephrasing it?
> > >
> > > My understanding was:
> > >  * we get this signal if there is a RAS error in the host memory
> > >  * if we are exposing RAS errors to the guest (ie we have
> > >    told it that in the ACPI table we passed it at startup)
> > >    then we should pass on this error to the guest
> > >
> > > but that these are two different conditions.
> > >
> > > If the host hardware detects a RAS error in memory used by the guest
> > > but the guest is not being told about RAS errors, then we cannot
> > > report the error: we have no mechanism to do so, and the guest is not
> > > expecting it.
> >
> > If you look at the x86 version of this function you can see that it tests 
> > (env->mcg_cap & MCG_SER_P), which I think is the equivalent x86 "is
> > the guest CPU/config one we can report these errors to" test.
>
> MCG_SER_P (software error recovery support present) flag indicates (when set) 
> that the processor supports software error recovery.
> env->mcg_cap 's value should be got from KVM as shown in the QEMU code[1], it 
> indicates whether the KVM support software error recovery.
>
> [1]:
> -------------------------------------------------------------------------------------------------------------
>   ret = kvm_get_mce_cap_supported(cs->kvm_state, &mcg_cap, &banks);
>   if (ret < 0) {
>        fprintf(stderr, "kvm_get_mce_cap_supported: %s", strerror(-ret));
>            return ret;
>    }
> -------------------------------------------------------------------------------------------------------------

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.

--------------------------------------------------------------------------------------------------------------
> void kvm_arch_on_sigbus_vcpu(CPUState *c, int code, void *addr)
> {
>         ...........................
>
>         if (ram_addr != RAM_ADDR_INVALID &&
>             kvm_physical_memory_addr_from_host(c->kvm_state, addr, &paddr)) {
>
>   If it got to here, it means the host hardware detects a RAS error in memory 
> used by the guest using above two judgments.
>   Maybe we can test/check whether KVM supports software error recovery in [3]
>

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.


thanks
-- PMM