Re: KVM 29: Page fault in kernel mode while booting GNU/kFreeBSD

[Aurelien Jarno <aurelien-rXXEIb44qovR7s880joybQ@public.gmane.org>]

On Wed, Jul 25, 2007 at 06:14:20AM +0300, Avi Kivity wrote:
> Aurelien Jarno wrote:
> > On Fri, Jul 20, 2007 at 09:57:55AM +0300, Avi Kivity wrote:
> >   
> >> Avi Kivity wrote:
> >>     
> >>>> Please tell me what I can do to help debugging this problem. 
> >>>> Alternatively I have uploaded an image to reproduce the problem here:
> >>>> http://temp.aurel32.net/kfreebsd-amd64.img.g
> >>>>     
> >>>>         
> >>> I've downloaded the image and booted it three times using kvm HEAD. 
> >>> Each time I got to the login prompt.
> >>>
> >>> Is this always reproducible?  Can you check with kvm HEAD (not that I
> >>> think of anything that could have fixed this).
> >>>
> >>>   
> >>>       
> >> Sorry, that was on Intel.  It reproduces easily on AMD.
> >>     
> >
> > By comparing the cases were the old and the new condition are different,
> > I have found that the problem occurs for the lmsw and smsw instructions.
> >   
> 
> Ah, I was looking at amd-specific instructions (vmx decodes lmsw) but
> missed these two.  Good work.
> 
> > The patch below is very ugly, but it workarounds the problem.
> >   
> 
> It's not ugly, it just lives in an ugly function.  But it does lack a
> changelog and a signoff.
> 

After a short discussion on IRC, it seems that all 0x02 0x01
instructions do not use the writeback. Please find an updated and
signedoff patch below.

  KVM: disable writeback for 0x02 0x01 instructions.

  0x02 0x01 instructions (ie lgdt, lidt, smsw, lmsw and invlpg) does
  not use writeback. This patch set no_wb=1 when emulating those 
  instructions.

  Signed-off-by: Aurelien Jarno <aurelien-rXXEIb44qovR7s880joybQ@public.gmane.org>

diff --git a/drivers/kvm/x86_emulate.c b/drivers/kvm/x86_emulate.c
index 21ce977..cbbb9c5 100644
--- a/drivers/kvm/x86_emulate.c
+++ b/drivers/kvm/x86_emulate.c
@@ -1183,6 +1183,8 @@ pop_instruction:
 twobyte_insn:
 	switch (b) {
 	case 0x01: /* lgdt, lidt, lmsw */
+		/* Disable writeback. */
+		no_wb = 1;
 		switch (modrm_reg) {
 			u16 size;
 			unsigned long address;
 

-- 
  .''`.  Aurelien Jarno	            | GPG: 1024D/F1BCDB73
 : :' :  Debian developer           | Electrical Engineer
 `. `'   aurel32-8fiUuRrzOP0dnm+yROfE0A@public.gmane.org         | aurelien-rXXEIb44qovR7s880joybQ@public.gmane.org
   `-    people.debian.org/~aurel32 | www.aurel32.net

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