On 09/12/2022 09:59, Roger Pau Monné wrote:
> On Thu, Dec 08, 2022 at 12:24:54PM +0100, Jan Beulich wrote:
>> core_set_legacy_ssbd() counts the number of times SSBD is being enabled
>> via LS_CFG on a core. This assumes that calls there only occur if the
>> state actually changes. While svm_ctxt_switch_{to,from}() conform to
>> this, guest_wrmsr() doesn't: It also calls the function when the bit
>> doesn't actually change. Extend the conditional there accordingly.
>>
>> Fixes: b2030e6730a2 ("amd/virt_ssbd: set SSBD at vCPU context switch")
>> Reported-by: Andrew Cooper <andrew.coop...@citrix.com>
>> Signed-off-by: Jan Beulich <jbeul...@suse.com>
>> ---
>> This is the less intrusive but more fragile variant of a fix. The
>> alternative would be to have core_set_legacy_ssbd() record per-thread
>> state, such that the necessary checking can be done there.
> Hm, yes, it's going to take a bit more of memory to keep track of
> this.It shouldn't. Turn the count field into a bitmap of thread_ids. The interface to this functionality should be WRMSR-like, and should support repeated writes of the same value. Anything else is a timebomb which has already gone off once... I'll have a play with this while looking into the repro I've got. ~Andrew
