Re: [kvm-devel] [PATCH] Refactor hypercall infrastructure (v2)

[Avi Kivity <avi@qumranet.com>]

Amit Shah wrote:
> * Anthony Liguori wrote:
>   
> > Amit Shah wrote:
> >     
> > > * Anthony Liguori wrote:
> > >  
> > >
> > >       
> > > > This patch refactors the current hypercall infrastructure to better
> > > > support live migration and SMP.  It eliminates the hypercall page by
> > > > trapping the UD exception that would occur if you used the wrong
> > > > hypercall instruction for the underlying architecture and replacing it
> > > > with the right one lazily. 
> > > >         
> > > This doesn't work right for SVM. It keeps looping indefinitely; on a
> > > kvm_stat run, I get about 230,000 light vm exits per second, with the
> > > hypercall never returning to the guest.
> > >
> > > ...
> > >  
> > >       
> > What are you using to issue the hypercall?
> >     
>
> +       r = kvm_hypercall1(KVM_PV_PCI_DEVICE, page_gfn);
>
> Setup is done by:
>
> +       if (!kvm_para_available()) {
> +               printk(KERN_ERR "KVM paravirt support not available\n");
> +               r = -ENODEV;
> +               goto out_dereg;
> +       }
>   

There was a bug where instructions with a modrm byte specifying a 
register would try to access memory.  In the memory was not mapped,  
emulation would fail. vmcall is one such instruction.  This was fixed by

commit f83562246921d6a8a7de8b76853a6835ace3699d
Author: Aurelien Jarno <aurelien@aurel32.net>
Date:   Wed Oct 17 19:30:41 2007 +0200

   KVM: x86 emulator: fix access registers for instructions with ModR/M 
byte and Mod = 3

   The patch belows changes the access type to register from memory for
   instructions that are declared as SrcMem or DstMem, but have a
   ModR/M byte with Mod = 3.

   It fixes (at least) the lmsw and smsw instructions on an AMD64 CPU,
   which are needed for FreeBSD.

   Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
   Signed-off-by: Avi Kivity <avi@qumranet.com>

diff --git a/drivers/kvm/x86_emulate.c b/drivers/kvm/x86_emulate.c
index 7c95ae5..8c50496 100644
--- a/drivers/kvm/x86_emulate.c
+++ b/drivers/kvm/x86_emulate.c
@@ -835,6 +835,14 @@ modrm_done:
               if (c->twobyte && c->b == 0x01
                                   && c->modrm_reg == 7)
                       break;
+               /*
+                * For instructions with a ModR/M byte, switch to register
+                * access if Mod = 3.
+                */
+               if ((c->d & ModRM) && c->modrm_mod == 3) {
+                       c->src.type = OP_REG;
+                       break;
+               }
srcmem_common:
               c->src.type = OP_MEM;
               break;
@@ -897,7 +905,14 @@ srcmem_common:
               }
               break;
       case DstMem:
-               c->dst.type = OP_MEM;
+               /*
+                * For instructions with a ModR/M byte, switch to register
+                * access if Mod = 3.
+                */
+               if ((c->d & ModRM) && c->modrm_mod == 3)
+                       c->dst.type = OP_REG;
+               else
+                       c->dst.type = OP_MEM;
               break;
       }




--
error compiling committee.c: too many arguments to function

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