Re: How to do fast accesses to LAPIC TPR under kvm?

[Avi Kivity <avi@redhat.com>]

On 10/24/2012 11:19 AM, Stefan Fritsch wrote:
>>
>> With the decode table fix I think it should work.
> 
> It needs some more changes. The patch below did the trick for me. It is
> against 3.5, because I didn't want to build a whole new kernel (my test
> machine is a dead slow AMD E-350).
> 
> The patch is definitely incomplete. It now allows the lock prefix for
> all mov operations on the cr1-7, which should not be the case. Apart
> from that, do the changes look reasonable? I have not checked that this
> is the minimal patch that works. But the LockReg bit was definitely
> necessary, that was the final piece to make it work.
> 
> Cheers,
> Stefan
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 4837375..c7f0ec7 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -128,6 +128,7 @@
>  #define Priv        (1<<27) /* instruction generates #GP if current CPL
> != 0 */
>  #define No64        (1<<28)
>  #define PageTable   (1 << 29)   /* instruction used to write page table */
> +#define LockReg     (1<<30) /* lock prefix is allowed for the
> instruction even for reg destination */
>  /* Source 2 operand type */
>  #define Src2Shift   (30)

LockReg conflicts with Src2Shift.

>  #define Src2None    (OpNone << Src2Shift)
> @@ -420,6 +421,7 @@ static int emulator_check_intercept(struct
> x86_emulate_ctxt *ctxt,
>      struct x86_instruction_info info = {
>          .intercept  = intercept,
>          .rep_prefix = ctxt->rep_prefix,
> +        .lock_prefix = ctxt->lock_prefix,
>          .modrm_mod  = ctxt->modrm_mod,
>          .modrm_reg  = ctxt->modrm_reg,
>          .modrm_rm   = ctxt->modrm_rm,
> @@ -2874,7 +2876,10 @@ static int em_mov(struct x86_emulate_ctxt *ctxt)
> 
>  static int em_cr_write(struct x86_emulate_ctxt *ctxt)
>  {
> -    if (ctxt->ops->set_cr(ctxt, ctxt->modrm_reg, ctxt->src.val))
> +    int cr = ctxt->modrm_reg;

Blank line here.

> +    if (ctxt->lock_prefix && cr == 0)
> +        cr = 8;

But maybe this is better dealt with during general decode, and
ctxt->modrm_reg adjusted instead.  This removes the code triplicstion.
Please also #UD if modrm_reg != 0, and if the feature is not exposed to
the guest via cpuid.

Please regenerate against kvm.git next, there have been changes to
emulate.c.

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