arm/arm64: KVM: Handle out-of-RAM cache maintenance as a NOP

So far, our handling of cache maintenance by VA has been pretty
simple: Either the access is in the guest RAM and generates a S2
fault, which results in the page being mapped RW, or we go down
the io_mem_abort() path, and nuke the guest.

The first one is fine, but the second one is extremely weird.
Treating the CM as an I/O is wrong, and nothing in the ARM ARM
indicates that we should generate a fault for something that
cannot end-up in the cache anyway (even if the guest maps it,
it will keep on faulting at stage-2 for emulation).

So let's just skip this instruction, and let the guest get away
with it.

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

diff --git a/arch/arm/include/asm/kvm_emulate.h b/arch/arm/include/asm/kvm_emulate.h
index 8a8c6de..ee5328f 100644 (file)
--- a/arch/arm/include/asm/kvm_emulate.h
+++ b/arch/arm/include/asm/kvm_emulate.h
@@ -138,6 +138,11 @@ static inline bool kvm_vcpu_dabt_iss1tw(struct kvm_vcpu *vcpu)
        return kvm_vcpu_get_hsr(vcpu) & HSR_DABT_S1PTW;
 }
 
+static inline bool kvm_vcpu_dabt_is_cm(struct kvm_vcpu *vcpu)
+{
+       return !!(kvm_vcpu_get_hsr(vcpu) & HSR_DABT_CM);
+}
+
 /* Get Access Size from a data abort */
 static inline int kvm_vcpu_dabt_get_as(struct kvm_vcpu *vcpu)
 {

diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index aba61fd..c3eb10e 100644 (file)
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -1431,6 +1431,22 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run)
                }
 
                /*
+                * Check for a cache maintenance operation. Since we
+                * ended-up here, we know it is outside of any memory
+                * slot. But we can't find out if that is for a device,
+                * or if the guest is just being stupid. The only thing
+                * we know for sure is that this range cannot be cached.
+                *
+                * So let's assume that the guest is just being
+                * cautious, and skip the instruction.
+                */
+               if (kvm_vcpu_dabt_is_cm(vcpu)) {
+                       kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
+                       ret = 1;
+                       goto out_unlock;
+               }
+
+               /*
                 * The IPA is reported as [MAX:12], so we need to
                 * complement it with the bottom 12 bits from the
                 * faulting VA. This is always 12 bits, irrespective

diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index 779a587..4df8e7a 100644 (file)
--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h
@@ -189,6 +189,11 @@ static inline bool kvm_vcpu_dabt_iss1tw(const struct kvm_vcpu *vcpu)
        return !!(kvm_vcpu_get_hsr(vcpu) & ESR_ELx_S1PTW);
 }
 
+static inline bool kvm_vcpu_dabt_is_cm(const struct kvm_vcpu *vcpu)
+{
+       return !!(kvm_vcpu_get_hsr(vcpu) & ESR_ELx_CM);
+}
+
 static inline int kvm_vcpu_dabt_get_as(const struct kvm_vcpu *vcpu)
 {
        return 1 << ((kvm_vcpu_get_hsr(vcpu) & ESR_ELx_SAS) >> ESR_ELx_SAS_SHIFT);