Re: [PATCH] always report x2apic as supported feature

[Gleb Natapov <gleb@redhat.com>]

On Thu, Jul 16, 2009 at 09:46:21AM +0800, Sheng Yang wrote:
> On Thursday 16 July 2009 07:01:30 Marcelo Tosatti wrote:
> > On Sun, Jul 12, 2009 at 04:10:55PM +0300, Gleb Natapov wrote:
> > > We emulate x2apic in software, so host support is not required.
> > >
> > > Signed-off-by: Gleb Natapov <gleb@redhat.com>
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 00844eb..c256da7 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -1497,6 +1497,9 @@ static void do_cpuid_ent(struct kvm_cpuid_entry2
> > > *entry, u32 function, case 1:
> > >  		entry->edx &= kvm_supported_word0_x86_features;
> > >  		entry->ecx &= kvm_supported_word4_x86_features;
> > > +		/* we support x2apic emulation even if host does not support
> > > +		   it since we emulate x2apic in software */
> > > +		entry->ecx |= F(X2APIC);
> > >  		break;
> > >  	/* function 2 entries are STATEFUL. That is, repeated cpuid commands
> > >  	 * may return different values. This forces us to get_cpu() before
> > > --
> > > 			Gleb.
> >
> > What if you have an older host that does not support emulate x2apic?
> 
> Due to interrupt remapping can't be enabled with KVM now, I think older host 
> would just ignore this info... (The new one can work without interrupt 
> remapping with KVM).
> 
> By the way, I saw X2APIC in host supported CPUID feature list(1.ecx), which I 
Where have you seen it? If you mean kvm_supported_word4_x86_features
then it is not what is supported by the host, but what is supported by
KVM. Host unsupported bits are dropped from there before reporting to
userspace. That is why this patch what necessary.

> don't think it's very properly. Host x2apic feature have nothing to do with 
> KVM, we do the emulation all the way. I suggest to remove the mask for host, 
> and give a comment that we would emulate all x2apic behaviour here, rather 
> than "even if", which I think it's a little misleading.
> 
> -- 
> regards
> Yang, Sheng

--
			Gleb.