Re: [Qemu-devel] [PATCH] i386: Add support to get/set/migrate MSR (33H)

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Jul 04, 2018 at 09:21:06PM +0800, Jingqi Liu wrote:
> The MSR (33H) controls support for #AC exception
> for split locked accesses. When bit 29 of the MSR (33H)
> is set, the processor causes an #AC exception to
> be issued instead of suppressing LOCK on bus
> (during split lock access).
> 
> Signed-off-by: Jingqi Liu <jingqi.liu@intel.com>
[...]
> diff --git a/target/i386/machine.c b/target/i386/machine.c
> index 4d98d36..c82dc0d 100644
> --- a/target/i386/machine.c
> +++ b/target/i386/machine.c
> @@ -935,6 +935,25 @@ static const VMStateDescription vmstate_msr_virt_ssbd = {
>      }
>  };
>  
> +static bool split_lock_ctrl_needed(void *opaque)
> +{
> +    X86CPU *cpu = opaque;
> +    CPUX86State *env = &cpu->env;
> +
> +    return env->split_lock_ctrl != 0;
> +}

Based on the Linux patch at [1], guests may try to detect the
feature by writing to the MSR unconditionally.

If this happens, KVM needs to provide a mechanism to
enable/disable the MSR emulation.  Otherwise users will end up
with VMs that can't be migrated to older hosts even if they are
using older machine-types.

[1] https://lkml.org/lkml/2018/6/29/408

-- 
Eduardo