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

Borislav Petkov
2015-11-23T16:43:24Z

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

#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.

> * KVM_MCE_CAP_SUPPORTED is MCG_CTL_P between
> 890ca9aefa78f7831f8f633cab9e4803636dffe4 (v2.6.32-rc1~693^2~199)
> and 5854dbca9b235f8cdd414a0961018763d2d5bf77 (v2.6.37-rc1~142^2~3)
>
> The current definitions in QEMU are:
> #define MCE_CAP_DEF (MCG_CTL_P|MCG_SER_P)
> #define MCE_BANKS_DEF 10

That's also wrong. The number of banks is, of course,
generation-specific. The qemu commit adding that is

79c4f6b08009 ("QEMU: MCE: Add MCE simulation to qemu/tcg")

and I really don't get what the reasoning behind it is? Is this supposed
to mimick a certain default CPU which is not really resembling a real
CPU or some generic CPU or what is it?

Because the moment you start qemu with -cpu <something AMD>, all that
MCA information is totally wrong.

> The target-i386/cpu.c:mce_init() code sets mcg_cap to:
> env->mcg_cap == MCE_CAP_DEF | MCE_BANKS_DEF;
> == (MCG_CTL_P|MCG_SER_P) | 10;
>
> The kvm_arch_init_vcpu() code that changes mcg_cap does the following:
> kvm_get_mce_cap_supported(cs->kvm_state, &mcg_cap, &banks);
> if (banks > MCE_BANKS_DEF) {
> banks = MCE_BANKS_DEF;
> }
> mcg_cap &= MCE_CAP_DEF;
> mcg_cap |= banks;
> env->mcg_cap = mcg_cap;
> * Therefore, if running Linux v2.6.37 or newer, this will be
> the result:
> banks == 10;
> mcg_cap == (MCG_CTL_P|MCG_SER_P) | banks
> == (MCG_CTL_P|MCG_SER_P) | 10;
> * That's the same value set by mce_init(), so fortunately
> kvm_arch_init_vcpu() isn't actually changing mcg_cap
> if running Linux v.2.6.37 and newer.
> * However, if running Linux between v2.6.32 and v2.6.37,
> kvm_arch_init_vcpu() will silently clear MCG_SER_P.

I don't understand - you want that cleared when emulating a !Intel CPU
or an Intel CPU which doesn't support SER. This bit should be clear
there. Why should be set at all there?

Hmmm?

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.