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

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2022/12/01 20:06, Marc Zyngier wrote:
> On Thu, 01 Dec 2022 10:49:11 +0000,
> Akihiko Odaki <akihiko.odaki@daynix.com> 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

> 
>> 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.

> 
>> 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. 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.

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.

Regards,
Akihiko Odaki

> 
> Do you mind having a look at this?
> 
> Thanks,
> 
> 	M.
>