Re: [PATCH] kvm: arm: Skip stage2 huge mappings for unaligned ipa backed by THP

[Andrew Murray]

On Tue, Apr 02, 2019 at 12:06:16PM +0100, Suzuki K Poulose wrote:
> With commit a80868f398554842b14, we no longer ensure that the
> THP page is properly aligned in the guest IPA. Skip the stage2
> huge mapping for unaligned IPA backed by transparent hugepages.
> 
> Fixes: a80868f398554842b14 ("KVM: arm/arm64: Enforce PTE mappings at stage2 when needed")
> Reported-by: Eric Auger <eric.auger@xxxxxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: Chirstoffer Dall <christoffer.dall@xxxxxxx>
> Cc: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> Cc: Zheng Xiang <zhengxiang9@xxxxxxxxxx>
> Tested-by: Eric Auger <eric.auger@xxxxxxx>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> ---
>  virt/kvm/arm/mmu.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index 27c9583..4a22f5b 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1412,7 +1412,9 @@ static bool transparent_hugepage_adjust(kvm_pfn_t *pfnp, phys_addr_t *ipap)
>  		 * page accordingly.
>  		 */
>  		mask = PTRS_PER_PMD - 1;
> -		VM_BUG_ON((gfn & mask) != (pfn & mask));
> +		/* Skip memslots with unaligned IPA and user address */
> +		if ((gfn & mask) != (pfn & mask))
> +			return false;
>  		if (pfn & mask) {
>  			*ipap &= PMD_MASK;
>  			kvm_release_pfn_clean(pfn);
> -- 
> 2.7.4

I was able to reproduce this issue on v5.1-rc3 on a SoftIron Overdrive 1000
(AMD Seattle (Rev.B1)) with defconfig+ARM64_64K_PAGES with:

qemu-system-aarch64 -cpu host -machine type=virt,accel=kvm -nographic -smp 4
-m 4096 -kernel /boot/vmlinuz-4.9.0-7-arm64 --append "console=ttyAMA0
default_hugepagesz=2M hugepages=256"

The 'default_hugepagesz=2M hugepages=256' had no effect on the reproducibility,
however the guest only intermittently failed to boot. Applying the above
patch fixed this and the guest boots every time.

Tested-by: Andrew Murray <andrew.murray@xxxxxxx>

Thanks,

Andrew Murray

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