Re: [PATCH 00/17] KVM: Use atomic SRCU for gfn-to-pfn cache, reinstate guest mode for x86 nesting

[David Woodhouse]

On Sun, 2026-09-20 at 21:49 +0100, David Woodhouse wrote:
> 
> There have been previous series which attempt to deal with various parts
> of GPC locking, and the steal time one has definitely been posted before
> a few times, but let's just call this v1:
> https://git.infradead.org/?p=users/dwmw2/linux.git;a=shortlog;h=refs/heads/gpc-srcu

I think I'd actually posted more of the previous RCU work than I
remembered, so perhaps this ought to have been a v4 of
https://lore.kernel.org/all/20260805195528.3853473-1-dwmw@xxxxxxxxxxxx/
but the entertaining parts of it probably stand alone now with the
atomic SRCU.

On the Sashiko feedback, much of it is down to lack of visibility of
atomic SRCU, and some parts are now fixed in my tree for what I shall
call v2 unless someone really objects to the above omission.

The interesting one is the EVTCHNOP_send deadlock, spinning on -EAGAIN
while in a kvm->srcu read section which prevents the memslot update
from ever completing. I've made it raise a REQ which completes it from
outside the read section, and added a selftest.

The MSR bitmap cache in patch 10 did lose the read-only mapping
support; I've added a readonly option to the GPC.

The iteration count in patch 17's commit message did indeed say 100k
where the code only does 10k. Fixed the former.

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