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

[gengdongjiu]

Hi Peter,
   Thanks for the comments.

> 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.

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.


> 
> --------------------------------------------------------------------------------------------------------------
> > 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.

[2]:
But QEMU should not know whether *guest* CPU / system support error reporting. 
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 

> 
> 
> thanks
> -- PMM