Re: [PATCH 0/3] KVM: arm64: Handle CCSIDR associativity mismatches

[Marc Zyngier]

On Thu, 01 Dec 2022 17:26:08 +0000,
Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> 
> On 2022/12/01 20:06, Marc Zyngier wrote:
> > On Thu, 01 Dec 2022 10:49:11 +0000,
> > Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> > 
> > Thanks for looking into this.
> > 
> >> M2 MacBook Air has mismatched CCSIDR associativity bits, which makes the
> >> bits a KVM vCPU sees inconsistent when migrating.
> > 
> > Can you describe the actual discrepancy? Is that an issue between the
> > two core types? In which case, nothing says that these two cluster
> > should have the same cache topology.
> 
> Yes, the processor has big.LITTLE configuration.
> 
> On the processor, the valid CSSELR values are 0 (L1D), 1 (L1I), 3
> (L2D). For each CSSELR values, each cluster has:
> - 0x700FE03A, 0x203FE01A, 0x70FFE07B
> - 0x701FE03A, 0x203FE02A, 0x73FFE07B

This is a perfectly valid configuration. The architecture doesn't
place any limitation on how different or identical the cache
hierarchies are from the PoV of each CPU. Actually, most big-little
systems show similar differences across their clusters.

> >> It also makes QEMU fail restoring the vCPU registers because QEMU saves
> >> and restores all of the registers including CCSIDRs, and if the vCPU
> >> migrated among physical CPUs between saving and restoring, it tries to
> >> restore CCSIDR values that mismatch with the current physical CPU, which
> >> causes EFAULT.
> > 
> > Well, QEMU will have plenty of other problems, starting with MIDRs,
> > which always reflect the physical one. In general, KVM isn't well
> > geared for VMs spanning multiple CPU types. It is improving, but there
> > is a long way to go.
> 
> On M2 MacBook Air, I have seen no other difference in standard ID
> registers and CCSIDRs are exceptions. Perhaps Apple designed this way
> so that macOS's Hypervisor can freely migrate vCPU, but I can't assure
> that without more analysis. This is still enough to migrate vCPU
> running Linux at least.

I guess that MacOS hides more of the underlying HW than KVM does. And
KVM definitely doesn't hide the MIDR_EL1 registers, which *are*
different between the two clusters.

> >> Trap CCSIDRs if there are CCSIDR value msimatches, and override the
> >> associativity bits when handling the trap.
> > 
> > TBH, I'd rather we stop reporting this stuff altogether.
> > 
> > There is nothing a correctly written arm64 guest should do with any of
> > this (this is only useful for set/way CMOs, which non-secure SW should
> > never issue). It would be a lot better to expose a virtual topology
> > (one set, one way, one level). It would also save us from the CCSIDRX
> > silliness.
> > 
> > The only complexity would be to still accept different topologies from
> > userspace so that we can restore a VM saved before this virtual
> > topology.
> 
> Another (minor) concern is that trapping relevant registers may cost
> too much. Currently KVM traps CSSELR and CCSIDR accesses with
> HCR_TID2, but HCR_TID2 also affects CTR_EL0.

It will have an additional impact (JITs, for example, will take a hit
if they don't cache that value), but this is pretty easy to mitigate
if it proves to have too much of an impact. We already have a bunch of
fast-paths for things that we want to emulate more efficiently, and
CTR_EL0 could be one of them,

> Although I'm not sure if the register is referred frequently, Arm
> introduced FEAT_EVT to trap CSSELR and CSSIDR but not CTR_EL0 so
> there may be some case where trapping CTR_EL0 is not
> tolerated. Perhaps Arm worried that a userspace application may read
> CTR_EL0 frequently.

FEAT_EVT is one of these "let's add random traps" extensions,
culminating in FEAT_FGT. Having FEAT_EVT would make it more efficient,
but we need to support this for all revisions of the architecture.

So let's first build on top of HCR_EL2.TID2, and only then once we
have an idea of the overhead add support for HCR_EL2.TID4 for the
systems that have FEAT_EVT.

> If you think the concern on VM restoration you mentioned and the
> trapping overhead is tolerable, I'll write a new, much smaller patch
> accordingly.

That would great, thanks. There are a number of gotchas around that
(like the 32bit stuff that already plays the emulation game), but this
is the right time to start and have something in 6.3 if you keep to it!

	M.

-- 
Without deviation from the norm, progress is not possible.