Re: [Xen-devel] [PATCH RFC] x86: Add hack to disable "Fake HT" mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.11.2019 16:23, George Dunlap wrote:
> On 11/15/19 2:59 PM, Andrew Cooper wrote:
>> On 15/11/2019 14:55, George Dunlap wrote:
>>>>> +            p->basic.htt = false;
>>>> I think everything below here indeed simply undoes what said old
>>>> commit did, but I can't match up this one. And together with the
>>>> question of whether instead leaving it alone, cmp_legacy then
>>>> would have the same question raised.
>>> This is based on a XenServer patch which reverts the entire commit, and
>>> has been maintained in the patchqueue since the commit made it upstream,
>>> AFAICT.  So I'll let someone from that team comment on the wherefores;
>>> but as I said, it's by far the best tested option we're going to get.
>>
>> Yes.  It is a revert of c/s ca2eee92df44 (Xen 3.4, and maintained
>> forwards until now) because it broke migration across that changeset.
>>
>> It is also this exact version of the revert which has been tested and
>> confirmed to fix the Ryzen 3xxx fixes.
>>
>> A separate experiment tried playing with only the flags, via
>> cpuid="host:htt=0,cmp_legacy=1" and this did not resolve the crashes.
> 
> Is that because the "revert"  still clears cmp_legacy, rather than
> setting it to 1?
> 
> I think what Jan was getting at was that ca2eee92df44 *sets* htt and
> *clears* cmp_legacy, but previous to that commit, htt and cmp_legacy
> weren't changed, they were simply left alone.  When reverting this
> patch, why do we not simply leave it alone, as was done before, rather
> than actively clearing them?

Actually no, I wasn't looking properly - HTT used to be cleared as much
as CMP_LEGACY before that change. Somehow I didn't spot the former when
putting together my earlier reply (maybe I looked for HTT when its only
HT there). So I'm sorry for the extra noise.

Jan

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