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

On 2020-10-01 4:30 a.m., Marc Zyngier wrote:

Hi Thomas,

On 2020-09-30 23:12, Thomas Tai wrote:

quoted

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.

quoted

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 <redacted>
---
 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

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