On Thu, May 08, 2025 at 10:08:32AM +0800, Yan Zhao wrote:
> On Wed, May 07, 2025 at 09:31:22AM -0700, Dave Hansen wrote:
> > On 5/5/25 05:44, Huang, Kai wrote:
> > >> +static int tdx_pamt_add(atomic_t *pamt_refcount, unsigned long hpa,
> > >> + struct list_head *pamt_pages)
> > >> +{
> > >> + u64 err;
> > >> +
> > >> + hpa = ALIGN_DOWN(hpa, SZ_2M);
> > >> +
> > >> + spin_lock(&pamt_lock);
> > > Just curious, Can the lock be per-2M-range?
> >
> > Folks, please keep it simple.
> >
> > If there's lock contention on this, we'll fix the lock contention, or
> > hash the physical address into a fixed number of locks. But having it be
> > per-2M-range sounds awful. Then you have to size it, and allocate it and
> > then resize it if there's ever hotplug, etc...
> In patch 2, there're per-2M-range pamt_refcounts. Could the per-2M-range
> lock be implemented in a similar way?
>
> +static atomic_t *pamt_refcounts;
> +atomic_t *tdx_get_pamt_refcount(unsigned long hpa)
> +{
> + return &pamt_refcounts[hpa / PMD_SIZE];
> +}
But why? If no contention, it is just wasteful.
> > Kirill, could you put together some kind of torture test for this,
> > please? I would imagine a workload which is sitting in a loop setting up
> > and tearing down VMs on a bunch of CPUs would do it.
> >
> > That ^ would be the worst possible case, I think. If you don't see lock
> > contention there, you'll hopefully never see it on real systems.
> When one vCPU is trying to install a guest page of HPA A, while another vCPU
> is trying to install a guest page of HPA B, theoretically they may content the
> global pamt_lock even if HPA A and B belong to different PAMT 2M blocks.
This contention will be be momentary if ever happen.
> > I *suspect* that real systems will get bottlenecked somewhere in the
> > page conversion process rather than on this lock. But it should be a
> > pretty simple experiment to run.
--
Kiryl Shutsemau / Kirill A. Shutemov
