Re: [Qemu-devel] [PATCH v7 1/9] i386: Helpers to encode cache information consistently

[Eduardo Habkost]

On Mon, May 07, 2018 at 09:14:27PM +0000, Moger, Babu wrote:
> Eduardo,
>    Thanks for all the comments. Will respond to each one separately. 
> 
> > -----Original Message-----
> > From: Eduardo Habkost [mailto:address@hidden
> > Sent: Monday, May 7, 2018 2:05 PM
> > To: Moger, Babu <address@hidden>
> > Cc: address@hidden; address@hidden; address@hidden;
> > address@hidden; address@hidden; address@hidden;
> > address@hidden; address@hidden; address@hidden
> > Subject: Re: [Qemu-devel] [PATCH v7 1/9] i386: Helpers to encode cache
> > information consistently
> > 
> > Hi,
> > 
> > I was about to apply this because I assumed it was the same patch
> > I sent in March, but then I found this:
> > 
> > On Thu, Apr 26, 2018 at 11:26:41AM -0500, Babu Moger wrote:
> > > From: Eduardo Habkost <address@hidden>
> > >
> > > Instead of having a collection of macros that need to be used in
> > > complex expressions to build CPUID data, define a CPUCacheInfo
> > > struct that can hold information about a given cache.  Helper
> > > functions will take a CPUCacheInfo struct as input to encode
> > > CPUID leaves for a cache.
> > >
> > > This will help us ensure consistency between cache information
> > > CPUID leaves, and make the existing inconsistencies in CPUID info
> > > more visible.
> > >
> > > Signed-off-by: Eduardo Habkost <address@hidden>
> > > Signed-off-by: Babu Moger <address@hidden>
> > > Tested-by: Geoffrey McRae <address@hidden>
> > [...]
> > > -#define L2_ASSOCIATIVITY      16
> > [...]
> > >  /*FIXME: CPUID leaf 0x80000006 is inconsistent with leaves 2 & 4 */
> > > +static CPUCacheInfo l2_cache_amd = {
> > [...]
> > > +    .associativity = 8,
> > [...]
> > > +};
> > [...]
> > >      case 0x80000006:
> > [...]
> > > -        *ecx = (L2_SIZE_KB_AMD << 16) | \
> > > -               (AMD_ENC_ASSOC(L2_ASSOCIATIVITY) << 12) | \
> > > -               (L2_LINES_PER_TAG << 8) | (L2_LINE_SIZE);
> > [...]
> > > +        encode_cache_cpuid80000006(&l2_cache_amd,
> > > +                                   cpu->enable_l3_cache ? &l3_cache : 
> > > NULL,
> > > +                                   ecx, edx);
> > [...]
> > 
> > The structs added by this patch are supposed to represent the
> > legacy cache sizes, and must match the old code.  My original
> > patch set l2_cache_amd.associativity=16 because of that.
> > 
> > This patch changes 0x80000006 from associativity=16 to
> > associativity=8.  Why?
> 
> The original code had a bug here.   The associativity should have been 8. 
> My earlier response from the thread
> http://patchwork.ozlabs.org/patch/884880/
> 
> This should have been 8-way. This is a bug. Will fix.
> This should have been (AMD_ENC_ASSOC(L2_ASSOCIATIVITY_AMD) << 12)

If we want to change the associativity, we must keep the old
values on older machine-types, which was the whole purpose of the
"legacy-cache" property.

I suggest using the new X86CPUDefinition::cache_info field if you
want to make AMD CPUs report different associativity.

-- 
Eduardo