[PATCH] KVM: arm64: Fixes ARM VM hangs at boot

[Thomas Tai]

On 2020-10-01 4:30 a.m., Marc Zyngier wrote:
> Hi Thomas,
> 
> On 2020-09-30 23:12, Thomas Tai wrote:
>> When using the latest kernel v5.9-rc7 on an ARM server, VMs could
>> hang at boot with no output. The commit a0e50aa3f4a8 removed
>> asm(ALTERNATIVE("isb", "nop", ARM64_WORKAROUND_SPECULATIVE_AT)),
>> it looks like isb is still needed for non
>> ARM64_WORKAROUND_SPECULATIVE_AT host when switching to guest.
> 
> Is that on an eMAG system, by any chance?

Hi Marc,
Yes, it is indeed an eMAG system.

> 
>>
>> The code fragment for the original patch:
>> -       /* __load_guest_stage2() includes an ISB for the workaround. */
>> -       __load_guest_stage2(kvm);
>> -       asm(ALTERNATIVE("isb", "nop", ARM64_WORKAROUND_SPECULATIVE_AT));
>> +       __load_guest_stage2(mmu);
>>
>> The code should have been:
>> -       /* __load_guest_stage2() includes an ISB for the workaround. */
>> -       __load_guest_stage2(kvm);
>> -       asm(ALTERNATIVE("isb", "nop", ARM64_WORKAROUND_SPECULATIVE_AT));
>> +       __load_guest_stage2(mmu);
>> +       asm(ALTERNATIVE("isb", "nop", ARM64_WORKAROUND_SPECULATIVE_AT));
>> ie, the workaround is still needed.
>>
>> Fixes: a0e50aa3f4a8 ("KVM: arm64: Factor out stage 2 page table data from
>> struct kvm")
>>
>> Signed-off-by: Thomas Tai <thomas.tai at oracle.com>
>> ---
>>  arch/arm64/kvm/hyp/nvhe/tlb.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/arm64/kvm/hyp/nvhe/tlb.c 
>> b/arch/arm64/kvm/hyp/nvhe/tlb.c
>> index 69eae60..536496e 100644
>> --- a/arch/arm64/kvm/hyp/nvhe/tlb.c
>> +++ b/arch/arm64/kvm/hyp/nvhe/tlb.c
>> @@ -32,6 +32,7 @@ static void __tlb_switch_to_guest(struct kvm_s2_mmu 
>> *mmu,
>>      }
>>
>>      __load_guest_stage2(mmu);
>> +    asm(ALTERNATIVE("isb", "nop", ARM64_WORKAROUND_SPECULATIVE_AT));
>>  }
>>
>>  static void __tlb_switch_to_host(struct tlb_inv_context *cxt)
> 
> Ouch. I remember fixing this a while ago, but somehow dropped it
> after applying it and *not* merging it...
> 
> Thanks a lot for the heads up!
> 
>          M.
> 
> [1] 
> https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/commit/?h=kvm-arm64/pre-nv-5.9 

Good to know you have the fix.

Thank you,
Thomas

>