Re: [Qemu-devel] [PATCH 1/2] Fix real mode guest migration

[Andreas Färber]

Am 22.07.2013 11:49, schrieb Paolo Bonzini:
> Il 22/07/2013 08:49, Orit Wasserman ha scritto:
>> Older KVM versions save CS dpl value to an invalid value for real mode guests
>> (0x3). This patch detect this situation when loading CPU state and set all 
>> the
>> segments dpl to zero.
>> This will allow migration from older KVM on host without unrestricted guest
>> to hosts with restricted guest support.
>> For example migration from a Penryn host (with kernel 2.6.32) to
>> a Westmere host.
>>
>> Signed-off-by: Orit Wasserman <address@hidden>
>> ---
>>  target-i386/machine.c | 18 ++++++++++++++++++
>>  1 file changed, 18 insertions(+)
>>
>> diff --git a/target-i386/machine.c b/target-i386/machine.c
>> index 3659db9..7e95829 100644
>> --- a/target-i386/machine.c
>> +++ b/target-i386/machine.c
>> @@ -260,6 +260,24 @@ static int cpu_post_load(void *opaque, int version_id)
>>      CPUX86State *env = &cpu->env;
>>      int i;
>>  
>> +    /*
>> +      Real mode guest segments register DPL should be zero.
>> +      Older KVM version were setting it worngly.
>> +      Fixing it will allow live migration from such host that don't have
>> +      restricted guest support to an host with unrestricted guest support
>> +      (otherwise the migration will fail with invalid guest state
>> +      error).
>> +    */
> 
> Coding standard asks for *s on every line.
> 
> As discussed offlist, I would prefer to have this in the kernel since
> that's where the bug is.  Gleb disagrees.
> 
> We need to find a third person who mediates...  Anthony, Eduardo, what
> do you think?

Having the code here does not look wrong to me, to enforce a consistent
state inside QEMU.

However I wonder what happens without this patch on Westmere? Might it
make sense to sanitize or at least "assert" (whatever the kernel
equivalent is ;)) in the ioctl setting X86CPU state to the vCPU that the
incoming values will be valid for the host CPU? And optionally in QEMU's
KVM code for the reverse direction, cpu_synchronize_state(), to cope
with older kernels?

Regards,
Andreas

>> +    if (!(env->cr[0] & CR0_PE_MASK) &&
>> +         (env->segs[R_CS].flags >> DESC_DPL_SHIFT & 3) != 0) {
>> +        env->segs[R_CS].flags &= ~(env->segs[R_CS].flags & DESC_DPL_MASK);
>> +        env->segs[R_DS].flags &= ~(env->segs[R_DS].flags & DESC_DPL_MASK);
>> +        env->segs[R_ES].flags &= ~(env->segs[R_ES].flags & DESC_DPL_MASK);
>> +        env->segs[R_FS].flags &= ~(env->segs[R_FS].flags & DESC_DPL_MASK);
>> +        env->segs[R_GS].flags &= ~(env->segs[R_GS].flags & DESC_DPL_MASK);
>> +        env->segs[R_SS].flags &= ~(env->segs[R_SS].flags & DESC_DPL_MASK);
>> +    }
>> +
>>      /* XXX: restore FPU round state */
>>      env->fpstt = (env->fpus_vmstate >> 11) & 7;
>>      env->fpus = env->fpus_vmstate & ~0x3800;

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