linux-kernel - Re: [PATCH] KVM: fix sil/dil/bpl/spl in the mod/rm fields

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Message-ID: <51A77FA8.6010007@redhat.com>
Date:	Thu, 30 May 2013 18:34:48 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	unlisted-recipients:; (no To-header on input)
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	gnatapov@...hat.com
Subject: Re: [PATCH] KVM: fix sil/dil/bpl/spl in the mod/rm fields

Il 30/05/2013 17:34, Paolo Bonzini ha scritto:
> Il 30/05/2013 16:35, Paolo Bonzini ha scritto:
>> The x86-64 extended low-byte registers were fetched correctly from reg,
>> but not from mod/rm.
>>
>> This fixes another bug in the boot of RHEL5.9 64-bit, but it is still
>> not enough.
> 
> Well, it is enough but it takes 2 minutes to reach the point where
> hardware virtualization is used.  It is doing a lot of stuff in
> emulation mode because FS and GS have leftovers from the A20 test:
> 
> FS =0000 0000000000000000 0000ffff 00009300 DPL=0 DS16 [-WA]
> GS =ffff 00000000000ffff0 0000ffff 00009300 DPL=0 DS16 [-WA]
> 
> 0x00000000000113be:  in     $0x92,%al
> 0x00000000000113c0:  or     $0x2,%al
> 0x00000000000113c2:  out    %al,$0x92
> 0x00000000000113c4:  xor    %ax,%ax
> 0x00000000000113c6:  mov    %ax,%fs
> 0x00000000000113c8:  dec    %ax
> 0x00000000000113c9:  mov    %ax,%gs
> 0x00000000000113cb:  inc    %ax
> 0x00000000000113cc:  mov    %ax,%fs:0x200
> 0x00000000000113d0:  cmp    %gs:0x210,%ax
> 0x00000000000113d5:  je     0x113cb
> 
> The DPL < RPL test fails.  Any ideas?  Should we introduce a new
> intermediate value for emulate_invalid_guest_state (0=none, 1=some, 2=full)?

One idea could be to replace invalid descriptors with NULL ones.  Then
you can intercept this in the #GP handler and trigger emulation for that
instruction only.

Paolo
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