On Sat, 2024-07-13 at 12:22 +0200, Paolo Bonzini wrote:
> On 7/13/24 03:38, Maxim Levitsky wrote:
> > 1. Getting rid of the segment cache. I am not sure how much it helps
> > these days - this code is very old.
> >
> > 2. Using a read/write lock - IMHO the cleanest solution but might
> > also affect performance.
>
> A read/write lock would cause a deadlock between the writer and the
> sched_out callback, since they run on the same CPU.
>
> I think the root cause of the issue is that clearing the cache should be
> done _after_ the writes (and should have a barrier() at the beginning,
> if only for cleanliness). So your patch 1 should leave the clearing of
> vmx->segment_cache.bitmask where it was.
>
> However, that would still leave an assumption: that it's okay that a
> sched_out during vmx_vcpu_reset() (or other functions that write segment
> data in the VMCS) accesses stale data, as long as the stale data is not
> used after vmx_vcpu_reset() returns. Your patch is a safer approach,
> but maybe wrap preempt_disable()/preempt_enable() with
>
> vmx_invalidate_segment_cache_start() {
> preempt_disable();
> }
> vmx_invalidate_segment_cache_end() {
> vmx->segment_cache.bitmask = 0;
> preempt_enable();
> }
>
> Paolo
>
Hi Paolo!
This looks like a very good idea, I'll do this in v2.
Thanks,
Best regards,
Maxim Levitsky
