On Thu, 18 Jul 2024 20:35:37 +0100, Ilias Stamatis wrote:
> The current MMIO coalescing design has a few drawbacks which limit its
> usefulness. Currently all coalesced MMIO zones use the same ring buffer.
> That means that upon a userspace exit we have to handle potentially
> unrelated MMIO writes synchronously. And a VM-wide lock needs to be
> taken in the kernel when an MMIO exit occurs.
>
> Additionally, there is no direct way for userspace to be notified about
> coalesced MMIO writes. If the next MMIO exit to userspace is when the
> ring buffer has filled then a substantial (and unbounded) amount of time
> may have passed since the first coalesced MMIO.
>
> [...]
Applied patch 1 to kvm-x86 generic. I deliberately didn't put this in fixes or
Cc: it for stable, as the bug has been around for sooo long without anyone
noticing that there's basically zero chance that the bug is actively causing
issues.
I also reworked and expanded the changelog significantly to make it more clear
why things break, what the fallout is (KVM can _sometimes_ use the full ring),
and to call out that the lockless scheme that the buggy commit was preparing
for never seems to have landed.
Please take a gander at the changelog and holler if I messed anything up.
[1/6] KVM: Fix coalesced_mmio_has_room() to avoid premature userspace exit
https://github.com/kvm-x86/linux/commit/92f6d4130497
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https://github.com/kvm-x86/linux/tree/next
