Re: [Qemu-devel] [PULL 11/13] target-i386: forward CPUID cache leaves when -cpu host is used

[Peter Lieven]

On 19.11.2013 15:57, Paolo Bonzini wrote:
> Il 19/11/2013 15:46, Peter Lieven ha scritto:
> > > Just replace "if (cs->nr_cores > 1)" in the patch I posted, i.e. after
> > > the switch.
> > This seems to work. What is in bits 0..5 of eax?
> It's the kind of cache.  0 means that there is no cache and the returned
> data is not valid.  In theory, Intel says you should only check whether
> those bits are 0, and stop iterating when they are, but apparently
> something is expecting eax=0.
>
> > What about the number of threads in count == 2?
> That's a property of the L2 cache.  It's not related to APIC IDs.
okay, but the contents could be wrong if the physical system
has threads while the emulated vserver has not. does this
matter?
> > I would still like to have at least an option to disable the passthru
> > without recompiling if other issues occur.
> I think of "-cpu host" in general as a hit-or-miss option.  I'm not sure
> whether it makes sense to have such fine grain.
As I explained, I have no trouble with -cpu host for ages when I ensure that
I do not live migrate to a processor that misses a feature that was there
when the vserver was created. With the caching option this could become
a problem or do you think wrong assumption about the cache are just
a performance penalty?

Peter