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Re: [Qemu-devel] [PATCH 1/2] Fix real mode guest migration
From: |
Anthony Liguori |
Subject: |
Re: [Qemu-devel] [PATCH 1/2] Fix real mode guest migration |
Date: |
Mon, 22 Jul 2013 12:50:23 -0500 |
User-agent: |
Notmuch/0.15.2+202~g0c4b8aa (http://notmuchmail.org) Emacs/23.3.1 (x86_64-pc-linux-gnu) |
Paolo Bonzini <address@hidden> writes:
> 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?
I can rationalize the post_load part. We may get garbage state via live
migration, we fix it up.
The pre_save part troubles me though. Is the kernel actively returning
invalid space to userspace? If so, this needs to be fixed. That's
clearly a bug.
Or is this expected to work around an old kernel? If so, is the kernel
advertising that it now handles this state correctly via a capability?
If not, it probably should.
Regards,
Anthony Liguori
>
> Paolo
>
>> + 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;
>>
- [Qemu-devel] [PATCH 2/2] Fix real mode guest segments dpl value in savevm, (continued)
- Re: [Qemu-devel] [PATCH 1/2] Fix real mode guest migration,
Anthony Liguori <=
Re: [Qemu-devel] [PATCH 1/2] Fix real mode guest migration, Juan Quintela, 2013/07/22
Re: [Qemu-devel] [PATCH 1/2] Fix real mode guest migration, Eric Blake, 2013/07/22