Re: MCG_CAP ABI breakage (was Re: [Qemu-devel] [PATCH] target-i386: Do not set MCG_SER_P by default)

Eduardo Habkost
2015-11-24T16:36:23Z

On Mon, Nov 23, 2015 at 05:43:14PM +0100, Borislav Petkov wrote:
> On Mon, Nov 23, 2015 at 01:11:27PM -0200, Eduardo Habkost wrote:
> > On Mon, Nov 23, 2015 at 11:22:37AM -0200, Eduardo Habkost wrote:
> > [...]
> > > In the case of this code, it looks like it's already broken
> > > because the resulting mcg_cap depends on host kernel capabilities
> > > (the ones reported by kvm_get_mce_cap_supported()), and the data
> > > initialized by target-i386/cpu.c:mce_init() is silently
> > > overwritten by kvm_arch_init_vcpu(). So we would need to fix that
> > > before implementing a proper compatibility mechanism for
> > > mcg_cap.
> >
> > Fortunately, when running Linux v2.6.37 and later,
> > kvm_arch_init_vcpu() won't actually change mcg_cap (see details
> > below).
> >
> > But the code is broken if running on Linux between v2.6.32 and
> > v2.6.36: it will clear MCG_SER_P silently (and silently enable
> > MCG_SER_P when migrating to a newer host).
> >
> > But I don't know what we should do on those cases. If we abort
> > initialization when the host doesn't support MCG_SER_P, all CPU
> > models with MCE and MCA enabled will become unrunnable on Linux
> > between v2.6.32 and v2.6.36. Should we do that, and simply ask
> > people to upgrade their kernels (or explicitly disable MCE) if
> > they want to run latest QEMU?
> >
> > For reference, these are the capabilities returned by Linux:
> > * KVM_MAX_MCE_BANKS is 32 since
> > 890ca9aefa78f7831f8f633cab9e4803636dffe4 (v2.6.32-rc1~693^2~199)
> > * KVM_MCE_CAP_SUPPORTED is (MCG_CTL_P | MCG_SER_P) since
> > 5854dbca9b235f8cdd414a0961018763d2d5bf77 (v2.6.37-rc1~142^2~3)
>
> The commit message of that one says that there is MCG_SER_P support in
> the kernel.
>
> The previous commit talks about MCE injection with KVM_X86_SET_MCE
> ioctl but frankly, I don't see that. From looking at the current code,
> KVM_X86_SET_MCE does kvm_vcpu_ioctl_x86_setup_mce() which simply sets
> MCG_CAP. And it gets those from KVM_X86_GET_MCE_CAP_SUPPORTED which is

KVM_X86_SET_MCE does not call kvm_vcpu_ioctl_x86_setup_mce(). It
calls kvm_vcpu_ioctl_x86_set_mce(), which stores the
IA32_MCi_{STATUS,ADDR,MISC} register contents at
vcpu->arch.mce_banks.

>
> #define KVM_MCE_CAP_SUPPORTED (MCG_CTL_P | MCG_SER_P)
>
> So it basically sets those two supported bits. But how is
>
> supported == actually present
>
> ?!?!
>
> That soo doesn't make any sense.
>

I didn't check the QEMU MCE code to confirm that, but I assume it
is implemented there. In that case, MCG_SER_P in
KVM_MCE_CAP_SUPPORTED just indicates it can be implemented by
userspace, as long as it makes the appropriate KVM_X86_SET_MCE
(or maybe KVM_SET_MSRS?) calls.

--
Eduardo