Re: [PATCH 01/10] KVM: arm/arm64: Split dcache/icache flushing

Christoffer Dall
2017-10-16T20:06:42Z

On Mon, Oct 09, 2017 at 04:20:23PM +0100, Marc Zyngier wrote:
> As we're about to introduce opportunistic invalidation of the icache,
> let's split dcache and icache flushing.

I'm a little confused abut the naming of these functions now,
because where I believe the current function ensures coherency between
the I-cache and D-cache (and overly so) if you just call one or the
other function after this change, what exactly is the coherency you get?

>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
> arch/arm/include/asm/kvm_mmu.h | 60 ++++++++++++++++++++++++++++------------
> arch/arm64/include/asm/kvm_mmu.h | 13 +++++++--
> virt/kvm/arm/mmu.c | 20 ++++++++++----
> 3 files changed, 67 insertions(+), 26 deletions(-)
>
> diff --git a/arch/arm/include/asm/kvm_mmu.h b/arch/arm/include/asm/kvm_mmu.h
> index fa6f2174276b..f553aa62d0c3 100644
> --- a/arch/arm/include/asm/kvm_mmu.h
> +++ b/arch/arm/include/asm/kvm_mmu.h
> @@ -126,21 +126,12 @@ static inline bool vcpu_has_cache_enabled(struct kvm_vcpu *vcpu)
> return (vcpu_cp15(vcpu, c1_SCTLR) & 0b101) == 0b101;
> }
>
> -static inline void __coherent_cache_guest_page(struct kvm_vcpu *vcpu,
> - kvm_pfn_t pfn,
> - unsigned long size)
> +static inline void __coherent_dcache_guest_page(struct kvm_vcpu *vcpu,
> + kvm_pfn_t pfn,
> + unsigned long size)
> {
> /*
> - * If we are going to insert an instruction page and the icache is
> - * either VIPT or PIPT, there is a potential problem where the host
> - * (or another VM) may have used the same page as this guest, and we
> - * read incorrect data from the icache. If we're using a PIPT cache,
> - * we can invalidate just that page, but if we are using a VIPT cache
> - * we need to invalidate the entire icache - damn shame - as written
> - * in the ARM ARM (DDI 0406C.b - Page B3-1393).
> - *
> - * VIVT caches are tagged using both the ASID and the VMID and doesn't
> - * need any kind of flushing (DDI 0406C.b - Page B3-1392).
> + * Clean the dcache to the Point of Coherency.
> *
> * We need to do this through a kernel mapping (using the
> * user-space mapping has proved to be the wrong
> @@ -155,19 +146,52 @@ static inline void __coherent_cache_guest_page(struct kvm_vcpu *vcpu,
>
> kvm_flush_dcache_to_poc(va, PAGE_SIZE);
>
> - if (icache_is_pipt())
> - __cpuc_coherent_user_range((unsigned long)va,
> - (unsigned long)va + PAGE_SIZE);
> -
> size -= PAGE_SIZE;
> pfn++;
>
> kunmap_atomic(va);
> }
> +}
>
> - if (!icache_is_pipt() && !icache_is_vivt_asid_tagged()) {
> +static inline void __coherent_icache_guest_page(struct kvm_vcpu *vcpu,
> + kvm_pfn_t pfn,
> + unsigned long size)
> +{
> + /*
> + * If we are going to insert an instruction page and the icache is
> + * either VIPT or PIPT, there is a potential problem where the host
> + * (or another VM) may have used the same page as this guest, and we
> + * read incorrect data from the icache. If we're using a PIPT cache,
> + * we can invalidate just that page, but if we are using a VIPT cache
> + * we need to invalidate the entire icache - damn shame - as written
> + * in the ARM ARM (DDI 0406C.b - Page B3-1393).
> + *
> + * VIVT caches are tagged using both the ASID and the VMID and doesn't
> + * need any kind of flushing (DDI 0406C.b - Page B3-1392).
> + */
> +
> + VM_BUG_ON(size & ~PAGE_MASK);
> +
> + if (icache_is_vivt_asid_tagged())
> + return;
> +
> + if (!icache_is_pipt()) {
> /* any kind of VIPT cache */
> __flush_icache_all();
> + return;
> + }
> +
> + /* PIPT cache. As for the d-side, use a temporary kernel mapping. */
> + while (size) {
> + void *va = kmap_atomic_pfn(pfn);
> +
> + __cpuc_coherent_user_range((unsigned long)va,
> + (unsigned long)va + PAGE_SIZE);
> +
> + size -= PAGE_SIZE;
> + pfn++;
> +
> + kunmap_atomic(va);
> }
> }
>
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index 672c8684d5c2..4c4cb4f0e34f 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -230,19 +230,26 @@ static inline bool vcpu_has_cache_enabled(struct kvm_vcpu *vcpu)
> return (vcpu_sys_reg(vcpu, SCTLR_EL1) & 0b101) == 0b101;
> }
>
> -static inline void __coherent_cache_guest_page(struct kvm_vcpu *vcpu,
> - kvm_pfn_t pfn,
> - unsigned long size)
> +static inline void __coherent_dcache_guest_page(struct kvm_vcpu *vcpu,
> + kvm_pfn_t pfn,
> + unsigned long size)
> {
> void *va = page_address(pfn_to_page(pfn));
>
> kvm_flush_dcache_to_poc(va, size);
> +}
>
> +static inline void __coherent_icache_guest_page(struct kvm_vcpu *vcpu,
> + kvm_pfn_t pfn,
> + unsigned long size)
> +{
> if (icache_is_aliasing()) {
> /* any kind of VIPT cache */
> __flush_icache_all();
> } else if (is_kernel_in_hyp_mode() || !icache_is_vpipt()) {
> /* PIPT or VPIPT at EL2 (see comment in __kvm_tlb_flush_vmid_ipa) */

unrelated: I went and read the comment in __kvm_tlb_flush_vmid_ipa, and
I don't really understand why there is only a need to flush the icache
if the host is running at EL1.

The text seems to describe the problem of remapping executable pages
within the guest. That seems to me would require icache maintenance of
the page that gets overwritten with new code, regardless of whether the
host runs at EL1 or EL2.

Of course it's easier done on VHE because we don't have to take a trap,
but the code seems to not invalidate the icache at all for VHE systems
that have VPIPT. I'm confused. Can you help?

> + void *va = page_address(pfn_to_page(pfn));
> +
> flush_icache_range((unsigned long)va,
> (unsigned long)va + size);
> }
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index b36945d49986..9e5628388af8 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1257,10 +1257,16 @@ void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,
> kvm_mmu_write_protect_pt_masked(kvm, slot, gfn_offset, mask);
> }
>
> -static void coherent_cache_guest_page(struct kvm_vcpu *vcpu, kvm_pfn_t pfn,
> - unsigned long size)
> +static void coherent_dcache_guest_page(struct kvm_vcpu *vcpu, kvm_pfn_t pfn,
> + unsigned long size)
> {
> - __coherent_cache_guest_page(vcpu, pfn, size);
> + __coherent_dcache_guest_page(vcpu, pfn, size);
> +}
> +
> +static void coherent_icache_guest_page(struct kvm_vcpu *vcpu, kvm_pfn_t pfn,
> + unsigned long size)
> +{
> + __coherent_icache_guest_page(vcpu, pfn, size);
> }
>
> static void kvm_send_hwpoison_signal(unsigned long address,
> @@ -1391,7 +1397,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> new_pmd = kvm_s2pmd_mkwrite(new_pmd);
> kvm_set_pfn_dirty(pfn);
> }
> - coherent_cache_guest_page(vcpu, pfn, PMD_SIZE);
> + coherent_dcache_guest_page(vcpu, pfn, PMD_SIZE);
> + coherent_icache_guest_page(vcpu, pfn, PMD_SIZE);
> +
> ret = stage2_set_pmd_huge(kvm, memcache, fault_ipa, &new_pmd);
> } else {
> pte_t new_pte = pfn_pte(pfn, mem_type);
> @@ -1401,7 +1409,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> kvm_set_pfn_dirty(pfn);
> mark_page_dirty(kvm, gfn);
> }
> - coherent_cache_guest_page(vcpu, pfn, PAGE_SIZE);
> + coherent_dcache_guest_page(vcpu, pfn, PAGE_SIZE);
> + coherent_icache_guest_page(vcpu, pfn, PAGE_SIZE);
> +
> ret = stage2_set_pte(kvm, memcache, fault_ipa, &new_pte, flags);
> }
>
> --
> 2.14.1
>

Otherwise this looks fine to me:

Acked-by: Christoffer Dall <cdall@linaro.org>

Thanks,
-Christoffer