[Qemu-devel] Re: [PATCH uq/master -v2 2/2] KVM, MCE, unpoison memory address across reboot

[Huang Ying <ying.huang@intel.com>]

On Wed, 2011-02-09 at 16:00 +0800, Jan Kiszka wrote:
> On 2011-02-09 04:00, Huang Ying wrote:
> > In Linux kernel HWPoison processing implementation, the virtual
> > address in processes mapping the error physical memory page is marked
> > as HWPoison.  So that, the further accessing to the virtual
> > address will kill corresponding processes with SIGBUS.
> > 
> > If the error physical memory page is used by a KVM guest, the SIGBUS
> > will be sent to QEMU, and QEMU will simulate a MCE to report that
> > memory error to the guest OS.  If the guest OS can not recover from
> > the error (for example, the page is accessed by kernel code), guest OS
> > will reboot the system.  But because the underlying host virtual
> > address backing the guest physical memory is still poisoned, if the
> > guest system accesses the corresponding guest physical memory even
> > after rebooting, the SIGBUS will still be sent to QEMU and MCE will be
> > simulated.  That is, guest system can not recover via rebooting.
> 
> Yeah, saw this already during my test...
> 
> > 
> > In fact, across rebooting, the contents of guest physical memory page
> > need not to be kept.  We can allocate a new host physical page to
> > back the corresponding guest physical address.
> 
> I just wondering what would be architecturally suboptimal if we simply
> remapped on SIGBUS directly. Would save us at least the bookkeeping.

Because we can not change the content of memory silently during guest OS
running, this may corrupts guest OS data structure and even ruins disk
contents.  But during rebooting, all guest OS state are discarded.

[snip]
> > @@ -1882,6 +1919,7 @@ int kvm_arch_on_sigbus_vcpu(CPUState *en
> >                  hardware_memory_error();
> >              }
> >          }
> > +        kvm_hwpoison_page_add(ram_addr);
> >  
> >          if (code == BUS_MCEERR_AR) {
> >              /* Fake an Intel architectural Data Load SRAR UCR */
> > @@ -1926,6 +1964,7 @@ int kvm_arch_on_sigbus(int code, void *a
> >                      "QEMU itself instead of guest system!: %p\n", addr);
> >              return 0;
> >          }
> > +        kvm_hwpoison_page_add(ram_addr);
> >          kvm_mce_inj_srao_memscrub2(first_cpu, paddr);
> >      } else
> >  #endif
> > 
> > 
> 
> Looks fine otherwise. Unless that simplification makes sense, I could
> offer to include this into my MCE rework (there is some minor conflict).
> If all goes well, that series should be posted during this week.

Thanks.

Best Regards,
Huang Ying