linux-kernel - Re: [PATCH v5 33/36] KVM: arm64: Wrap the host with a stage 2

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Message-ID: <YFCqYT3Ib8kAN7/c@google.com>
Date:   Tue, 16 Mar 2021 12:53:53 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Mate Toth-Pal <mate.toth-pal@....com>
Cc:     catalin.marinas@....com, will@...nel.org, maz@...nel.org,
        james.morse@....com, julien.thierry.kdev@...il.com,
        suzuki.poulose@....com, android-kvm@...gle.com, seanjc@...gle.com,
        linux-kernel@...r.kernel.org, robh+dt@...nel.org,
        linux-arm-kernel@...ts.infradead.org, kernel-team@...roid.com,
        kvmarm@...ts.cs.columbia.edu, tabba@...gle.com, ardb@...nel.org,
        mark.rutland@....com, dbrazdil@...gle.com
Subject: Re: [PATCH v5 33/36] KVM: arm64: Wrap the host with a stage 2

On Tuesday 16 Mar 2021 at 13:28:42 (+0100), Mate Toth-Pal wrote:
> Testing the latest version of the patchset, we seem to have found another
> thing related to FEAT_S2FWB.

Argh! I wish I could put my hands on hardware with FWB. Thanks again for
the report.

> This function always sets Normal memory in the stage 2 table, even if the
> address in stage 1 was mapped as a device memory. However with the current
> settings for normal memory (i.e. MT_S2_FWB_NORMAL being defined to 6)
> according to the architecture (See Arm ARM, 'D5.5.5 Stage 2 memory region
> type and cacheability attributes when FEAT_S2FWB is implemented') the
> resulting attributes will be 'Normal Write-Back' even if the stage 1 mapping
> sets device memory. Accessing device memory mapped like this causes an
> SError on some platforms with FEAT_S2FWB being implemented.

Right.

> Changing the value of MT_S2_FWB_NORMAL to 7 would change this behavior, and
> the resulting memory type would be device.

Sounds like the correct fix here -- see below.

> Another solution would be to add an else branch to the last 'if' above like
> this:
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index fffa432ce3eb..54e5d3b0b2e1 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -214,6 +214,8 @@ static int host_stage2_idmap(u64 addr)
> 
>         if (is_memory)
>                 prot |= KVM_PGTABLE_PROT_X;
> +       else
> +               prot |= KVM_PGTABLE_PROT_DEVICE;
> 
>         hyp_spin_lock(&host_kvm.lock);
>         ret = kvm_pgtable_stage2_find_range(&host_kvm.pgt, addr, prot,
> &range);

While this would work in this particular case, I don't think we should
force all non-RAM accesses as device as the host may have reasons not to
want this (e.g. accessing SRAM). Your first suggestions allows us to do
just that, so it is preferable I think.

Thanks,
Quentin