On 27/11/2018 09:50, James Morse wrote:
> Hi Marc,
>
> On 23/11/2018 18:41, Marc Zyngier wrote:
>> In order to avoid TLB corruption whilst invalidating TLBs on CPUs
>> affected by erratum 1165522, we need to prevent S1 page tables
>> from being usable.
>>
>> For this, we set the EL1 S1 MMU on, and also disable the page table
>> walker (by setting the TCR_EL1.EPD* bits to 1).
>>
>> This ensures that once we switch to the EL1/EL0 translation regime,
>> speculated AT instructions won't be able to parse the page tables.
>
> Reviewed-by: James Morse <[email protected]>
>
>
> I think we can ditch an isb or two:
>
>> diff --git a/arch/arm64/kvm/hyp/tlb.c b/arch/arm64/kvm/hyp/tlb.c
>> index 7fcc9c1a5f45..0506ced16afc 100644
>> --- a/arch/arm64/kvm/hyp/tlb.c
>> +++ b/arch/arm64/kvm/hyp/tlb.c
>> @@ -21,12 +21,37 @@
>> #include <asm/kvm_mmu.h>
>> #include <asm/tlbflush.h>
>>
>> +struct tlb_inv_context {
>> + unsigned long flags;
>> + u64 tcr;
>> + u64 sctlr;
>> +};
>> +
>> static void __hyp_text __tlb_switch_to_guest_vhe(struct kvm *kvm,
>> - unsigned long *flags)
>> + struct tlb_inv_context *cxt)
>> {
>> u64 val;
>>
>> - local_irq_save(*flags);
>> + local_irq_save(cxt->flags);
>> +
>> + if (cpus_have_const_cap(ARM64_WORKAROUND_1165522)) {
>> + /*
>> + * For CPUs that are affected by ARM erratum 1165522, we
>> + * cannot trust stage-1 to be in a correct state at that
>> + * point. Since we do not want to force a full load of the
>> + * vcpu state, we prevent the EL1 page-table walker to
>> + * allocate new TLBs. This is done by setting the EPD bits
>> + * in the TCR_EL1 register. We also need to prevent it to
>> + * allocate API->PA walks, so we enable the S1 MMU...
>
> typo: API => IPA
>
>
>> + */
>> + val = cxt->tcr = read_sysreg_el1(tcr);
>> + val |= TCR_EPD1_MASK | TCR_EPD0_MASK;
>> + write_sysreg_el1(val, tcr);
>> + val = cxt->sctlr = read_sysreg_el1(sctlr);
>> + val |= SCTLR_ELx_M;
>> + write_sysreg_el1(val, sctlr);
>
>> + isb();
>
> Could you leave these to be synchronised by the isb() in
> __load_guest_stage2()?
> An AT speculated here would see HCR_EL2.TGE set and use the EL2&EL0 regime.
Yes, you're right.
>
>> + }
>>
>> /*
>> * With VHE enabled, we have HCR_EL2.{E2H,TGE} = {1,1}, and
>> @@ -34,8 +59,13 @@ static void __hyp_text __tlb_switch_to_guest_vhe(struct
>> kvm *kvm,
>> * guest TLBs (EL1/EL0), we need to change one of these two
>> * bits. Changing E2H is impossible (goodbye TTBR1_EL2), so
>> * let's flip TGE before executing the TLB operation.
>> + *
>> + * ARM erratum 1165522 requires some special handling (again),
>
>> + * as we need to make sure stage-2 is in place before clearing
>> + * TGE.
>
> Typo: stage-1?
>
> stage2 remains disabled here, we only call __load_guest_stage2() for the vmid.
> The problem was the EL1:stage-1 being usable and unknown when TGE is cleared.
>
>
>> */
>> __load_guest_stage2(kvm);
>> + asm(ALTERNATIVE("nop", "isb", ARM64_WORKAROUND_1165522));
>
> __load_guest_stage2() already has an isb for this workaround after it writes
> vtcr/vttbr. I think we can just refer to it in the comment and let it
> synchronise the stage1+2 config before we touch hcr_el2 below.
Well spotted, I've dropped that line and hopefully clarified the comment.
>
>
>> val = read_sysreg(hcr_el2);
>> val &= ~HCR_TGE;
>> write_sysreg(val, hcr_el2);
>
> [...]
>
>> @@ -64,11 +94,19 @@ static void __hyp_text __tlb_switch_to_host_vhe(struct
>> kvm *kvm,
>> write_sysreg(0, vttbr_el2);
>> write_sysreg(HCR_HOST_VHE_FLAGS, hcr_el2);
>> isb();
>> - local_irq_restore(flags);
>> +
>> + if (cpus_have_const_cap(ARM64_WORKAROUND_1165522)) {
>> + /* Restore the guest's registers to what they were */
>> + write_sysreg_el1(cxt->tcr, tcr);
>> + write_sysreg_el1(cxt->sctlr, sctlr);
>
>> + isb();
>
> Hmm, why do we need this isb?
>
> We just set TGE, so these registers values no long matter. vcpu_put() would
> read
> the values we wrote, as would __tlb_switch_to_guest_vhe() above if we re-ran
> this sequence. If we're on our way into the guest, the extra isb in
> __load_guest_stage2() would synchronise them before clearing TGE during
> world-switch.
>
> I don't think there is a path where we depend on these values being isb'd
> before
> guest eret.
Absolutely right, I dropped that one to. Yeah, 3 ISB down, profit! ;-)
> (if its just to be robust, I'm all in favour of it!)
No, it was all about being paranoid, thanks for curing that for me!
M.
--
Jazz is not dead. It just smells funny...
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