Re: [PATCHv14 5/9] efi: Add unaccepted memory support

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Fri, Oct 13, 2023 at 12:45:20PM -0500, Tom Lendacky wrote:
> On 10/13/23 11:22, Kirill A. Shutemov wrote:
> > On Fri, Oct 13, 2023 at 03:33:58PM +0300, Kirill A. Shutemov wrote:
> > > > While testing SNP guests running today's tip/master (ef19bc9dddc3) I ran
> > > > into what seems to be fairly significant lock contention due to the
> > > > unaccepted_memory_lock spinlock above, which results in a constant stream
> > > > of soft-lockups until the workload gets all its memory accepted/faulted
> > > > in if the guest has around 16+ vCPUs.
> > > > 
> > > > I've included the guest dmesg traces I was seeing below.
> > > > 
> > > > In this case I was running a 32 vCPU guest with 200GB of memory running on
> > > > a 256 thread EPYC (Milan) system, and can trigger the above situation fairly
> > > > reliably by running the following workload in a freshly-booted guests:
> > > > 
> > > >    stress --vm 32 --vm-bytes 5G --vm-keep
> > > > 
> > > > Scaling up the number of stress threads and vCPUs should make it easier
> > > > to reproduce.
> > > > 
> > > > Other than unresponsiveness/lockup messages until the memory is accepted,
> > > > the guest seems to continue running fine, but for large guests where
> > > > unaccepted memory is more likely to be useful, it seems like it could be
> > > > an issue, especially when consider 100+ vCPU guests.
> > > 
> > > Okay, sorry for delay. It took time to reproduce it with TDX.
> > > 
> > > I will look what can be done.
> > 
> > Could you check if the patch below helps?
> > 
> > diff --git a/drivers/firmware/efi/unaccepted_memory.c b/drivers/firmware/efi/unaccepted_memory.c
> > index 853f7dc3c21d..591da3f368fa 100644
> > --- a/drivers/firmware/efi/unaccepted_memory.c
> > +++ b/drivers/firmware/efi/unaccepted_memory.c
> > @@ -8,6 +8,14 @@
> >   /* Protects unaccepted memory bitmap */
> >   static DEFINE_SPINLOCK(unaccepted_memory_lock);
> > +struct accept_range {
> > +	struct list_head list;
> > +	unsigned long start;
> > +	unsigned long end;
> > +};
> > +
> > +static LIST_HEAD(accepting_list);
> > +
> >   /*
> >    * accept_memory() -- Consult bitmap and accept the memory if needed.
> >    *
> > @@ -24,6 +32,7 @@ void accept_memory(phys_addr_t start, phys_addr_t end)
> >   {
> >   	struct efi_unaccepted_memory *unaccepted;
> >   	unsigned long range_start, range_end;
> > +	struct accept_range range, *entry;
> >   	unsigned long flags;
> >   	u64 unit_size;
> > @@ -80,7 +89,25 @@ void accept_memory(phys_addr_t start, phys_addr_t end)
> >   	range_start = start / unit_size;
> > +	range.start = start;
> > +	range.end = end;
> > +retry:
> >   	spin_lock_irqsave(&unaccepted_memory_lock, flags);
> > +
> > +	list_for_each_entry(entry, &accepting_list, list) {
> > +		if (entry->end < start)
> > +			continue;
> > +		if (entry->start > end)
> 
> Should this be a >= check since start and end are page aligned values?

Right. Good catch.

> > +			continue;
> > +		spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
> > +
> > +		/* Somebody else accepting the range */
> > +		cpu_relax();
> > +		goto retry;
> 
> Could you set some kind of flag here so that ...
> 
> > +	}
> > +
> 
> ... once you get here, that means that area was accepted and removed from
> the list, so I think you could just drop the lock and exit now, right?
> Because at that point the bitmap will have been updated and you wouldn't be
> accepting any memory anyway?

No. Consider the case if someone else accept part of the range you are
accepting.

I guess we can check if the range on the list covers what we are accepting
fully, but it complication. Checking bitmap at this point is cheap enough:
we already hold the lock.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov