Re: [Xen-devel] [PATCH] tools/libxc: Properly increment ApicIdCoreSize field on AMD

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Jul 22, 2016 at 01:45:05PM -0400, Boris Ostrovsky wrote:
> On 07/22/2016 01:38 PM, Wei Liu wrote:
> > On Fri, Jul 22, 2016 at 01:14:01PM -0400, Boris Ostrovsky wrote:
> >> Current code incorrectly adds 1 to full register instead of
> >> incrementing the field in bits 15:12.
> >>
> >> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> >
> > I trust your expertise in this field. :-)
> 
> 
> Just in this field? The field is only 4 bits! ;-)
> 

No, it's only one bit! :-P

> But this actually fixes a regression that was triggered by recent
> hvmloader change on one particular processor that we have in the test farm.

Ian, this is a backport candidate and needs to go back as far as
possible. The bogus calculation was introduced in 2008 (!). 

Wei.

> 
> -boris
> 
> >
> >> ---
> >>  tools/libxc/xc_cpuid_x86.c | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/tools/libxc/xc_cpuid_x86.c b/tools/libxc/xc_cpuid_x86.c
> >> index 84f4e08..fbbac9e 100644
> >> --- a/tools/libxc/xc_cpuid_x86.c
> >> +++ b/tools/libxc/xc_cpuid_x86.c
> >> @@ -331,7 +331,8 @@ static void amd_xc_cpuid_policy(xc_interface *xch,
> >>           * ECX[15:12] is ApicIdCoreSize: ECX[7:0] is NumberOfCores (minus 
> >> one).
> >>           * Update to reflect vLAPIC_ID = vCPU_ID * 2.
> >>           */
> >> -        regs[2] = ((regs[2] & 0xf000u) + 1) | ((regs[2] & 0xffu) << 1) | 
> >> 1u;
> >> +        regs[2] = ((regs[2] + (1u << 12)) & 0xf000u) |
> >> +                  ((regs[2] & 0xffu) << 1) | 1u;
> >>          break;
> >>  
> >>      case 0x8000000a: {
> >> -- 
> >> 1.8.3.1
> >>
> 

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