Re: [RFC PATCH 6/7] arm64: KVM: Handle trappable TLB instructions

Punit Agrawal
2016-08-24T10:40:39Z

Will Deacon <will.deacon@arm.com> writes:

> Hi Punit,
>
> On Tue, Aug 16, 2016 at 11:45:11AM +0100, Punit Agrawal wrote:
>> The ARMv8 architecture allows trapping of TLB maintenane instructions
>> from EL0/EL1 to higher exception levels. On encountering a trappable TLB
>> instruction in a guest, an exception is taken to EL2.
>>
>> Add functionality to handle emulating the TLB instructions.
>>
>> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
>> Cc: Christoffer Dall <christoffer.dall@linaro.org>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>
> [...]
>
>> +void __hyp_text
>> +__kvm_emulate_tlb_invalidate(struct kvm *kvm, u32 sys_op, u64 regval)
>> +{
>> + kvm = kern_hyp_va(kvm);
>> +
>> + /*
>> + * Switch to the guest before performing any TLB operations to
>> + * target the appropriate VMID
>> + */
>> + __switch_to_guest_regime(kvm);
>> +
>> + /*
>> + * TLB maintenance operations broadcast to inner-shareable
>> + * domain when HCR_FB is set (default for KVM).
>> + */
>> + switch (sys_op) {
>> + case TLBIALL:
>> + case TLBIALLIS:
>> + case ITLBIALL:
>> + case DTLBIALL:
>> + case TLBI_VMALLE1:
>> + case TLBI_VMALLE1IS:
>> + __tlbi(vmalle1is);
>> + break;
>> + case TLBIMVA:
>> + case TLBIMVAIS:
>> + case ITLBIMVA:
>> + case DTLBIMVA:
>> + case TLBI_VAE1:
>> + case TLBI_VAE1IS:
>> + __tlbi(vae1is, regval);
>
> I'm pretty nervous about this. Although you've switched in the guest stage-2
> page table before the TLB maintenance, we're still running on a host stage-1
> and it's not clear to me that the stage-1 context is completely ignored for
> the purposes of a stage-1 TLBI executed at EL2.
>
> For example, if TCR_EL1.TBI0 is set in the guest but cleared in the host,
> my reading of the architecture is that it will be treated as zero when
> we perform this invalidation operation. I worry that we have similar
> problems with the granule size, where bits become RES0 in the TLBI VA
> ops.

Some control bits seem to be explicitly called out to not affect TLB
maintenance operations[0] but I hadn't considered the ones you highlight.

[0] ARMv8 ARM DDI 0487A.j D4.7, Pg D4-1814

>
> Finally, we should probably be masking out the RES0 bits in the TLBI
> ops, just in case some future extension to the architecture defines them
> in such a way where they have different meanings when executed at EL2
> or EL1.

Although, the RES0 bits for TLBI VA ops are currently ignored, I agree
that masking them out based on granule size protects against future
incompatible changes.

>
> The easiest thing to do is just TLBI VMALLE1IS for all trapped operations,
> but you might want to see how that performs.

That sounds reasonable for correctness. But I suspect we'll have to do
more to claw back some performance. Let me run a few tests and come back
on this.

Thanks for having a look.

Punit

>
> Will
> _______________________________________________
> kvmarm mailing list
> kvmarm@lists.cs.columbia.edu
> https://lists.cs.columbia.edu/mailman/listinfo/kvmarm