Re: [PATCH v1 2/2] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory

Will Deacon
2023-10-12T12:27:41Z

On Thu, Sep 07, 2023 at 11:14:59AM -0700, ankita@nvidia.com wrote:
> From: Ankit Agrawal <ankita@nvidia.com>
>
> Linux allows device drivers to map IO memory on a per-page basis using
> "write combining" or WC. This is often done using
> pgprot_writecombing(). The driver knows which pages can support WC
> access and the proper programming model to generate this IO. Generally
> the use case is to boost performance by using write combining to
> generate larger PCIe MemWr TLPs.
>
> Allow VMs to select DEVICE_* or NORMAL_NC on a page by page basis for
> all IO memory. This puts the VM in charge of the memory attributes,
> and removes the KVM override to DEVICE_nGnRE.
>
> Ultimately this makes pgprot_writecombing() work correctly in VMs and
> allows drivers like mlx5 to fully operate their HW.
>
> After some discussions with ARM and CPU architects we reached the
> conclusion there was no need for KVM to prevent the VM from selecting
> between DEVICE_* and NORMAL_NC for IO memory in VMs. There was a fear
> that NORMAL_NC could result in uncontained failures, but upon deeper
> analysis it turns out there are already possible cases for uncontained
> failures with DEVICE types too. Ultimately the platform must be
> implemented in a way that ensures that all DEVICE_* and NORMAL_NC
> accesses have no uncontained failures.
>
> Fortunately real platforms do tend to implement this.
>
> This patch makes the VM's memory attributes behave as follows:
>
>  S1           |   S2          |  Result
>  NORMAL-WB    |  NORMAL-NC    |  NORMAL-NC
>  NORMAL-WT    |  NORMAL-NC    |  NORMAL-NC
>  NORMAL-NC    |  NORMAL-NC    |  NORMAL-NC
>  DEVICE<attr> |  NORMAL-NC    |  DEVICE<attr>
>
> See section D8.5.5 of DDI0487_I_a_a-profile_architecture_reference_manual.pdf
> for details.
>
> Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
> ---
> arch/arm64/include/asm/memory.h | 2 ++
> arch/arm64/kvm/hyp/pgtable.c | 2 +-
> 2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index fde4186cc387..c247e5f29d5a 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -147,6 +147,7 @@
> * Memory types for Stage-2 translation
> */
> #define MT_S2_NORMAL 0xf
> +#define MT_S2_NORMAL_NC 0x5
> #define MT_S2_DEVICE_nGnRE 0x1
>
> /*
> @@ -154,6 +155,7 @@
> * Stage-2 enforces Normal-WB and Device-nGnRE
> */
> #define MT_S2_FWB_NORMAL 6
> +#define MT_S2_FWB_NORMAL_NC 5
> #define MT_S2_FWB_DEVICE_nGnRE 1
>
> #ifdef CONFIG_ARM64_4K_PAGES
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index ccd291b6893d..a80949002191 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -696,7 +696,7 @@ static int stage2_set_prot_attr(struct kvm_pgtable *pgt, enum kvm_pgtable_prot p
> kvm_pte_t *ptep)
> {
> bool device = prot & KVM_PGTABLE_PROT_DEVICE;
> - kvm_pte_t attr = device ? KVM_S2_MEMATTR(pgt, DEVICE_nGnRE) :
> + kvm_pte_t attr = device ? KVM_S2_MEMATTR(pgt, NORMAL_NC) :
> KVM_S2_MEMATTR(pgt, NORMAL);
> u32 sh = KVM_PTE_LEAF_ATTR_LO_S2_SH_IS;

I think this is putting the policy into the low-level page-table code,
which isn't where it belongs. KVM_PGTABLE_PROT_DEVICE means that the
mapping should be created with device attributes. If you want other
attributes, then please introduce another entry to 'kvm_pgtable_prot'
and pass that instead (e.g. KVM_PGTABLE_PROT_NC, which coincidentally
we already have in Android [1] ;).

Will

[1] https://android.googlesource.com/kernel/common/+/72cc19df8b71095f9740ff0ca6a75bf7ed27b0cd%5E%21/