Re: [RFC PATCH v2 00/26] KVM/arm64: A stage 2 for the host

[Mate Toth-Pal]

Hi Quentin,


On 2021-01-08 13:14, Quentin Perret wrote:
> Hi all,
>
> This is the v2 of the series previously posted here:
>
>    https://lore.kernel.org/kvmarm/20201117181607.1761516-1-qperret@xxxxxxxxxx/
>
> This basically allows us to wrap the host with a stage 2 when running in
> nVHE, hence paving the way for protecting guest memory from the host in
> the future (among other use-cases). For more details about the
> motivation and the design angle taken here, I would recommend to have a
> look at the cover letter of v1, and/or to watch these presentations at
> LPC [1] and KVM forum 2020 [2].


We tested the pKVM changes pulled from here:


>      https://android-kvm.googlesource.com/linux qperret/host-stage2-v2


We were using a target with Arm architecture with FEAT_S2FWB, and found 
that there is a bug in the patch.


It turned out that the Kernel checks for the extension, and sets up the 
stage 2 translation so that it forces the host memory type to 
write-through. However it seems that the code doesn't turn on the 
feature in the HCR_EL2 register.


We were able to fix the issue by applying the following patch:


diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c 
b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index 0cd3eb178f3b..e8521a072ea6 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -105,6 +105,8 @@ int kvm_host_prepare_stage2(void *mem_pgt_pool, void 
*dev_pgt_pool)
                params->vttbr = kvm_get_vttbr(mmu);
                params->vtcr = host_kvm.arch.vtcr;
                params->hcr_el2 |= HCR_VM;
+               if (cpus_have_const_cap(ARM64_HAS_STAGE2_FWB))
+                       params->hcr_el2 |= HCR_FWB;
                __flush_dcache_area(params, sizeof(*params));
        }


Best regards,
Mate Toth-Pal