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

[Marc Zyngier]

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?

> 
> 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
-- 
Jazz is not dead. It just smells funny...