Re: [PATCH 05/16] KVM: arm64: Plumb MMIO checking into the fault handling

[Will Deacon]

On Thu, Jul 15, 2021 at 05:31:48PM +0100, Marc Zyngier wrote:
> Plumb the MMIO checking code into the MMIO fault handling code.
> Nothing allows a region to be registered yet, so there should be
> no funtional change either.

Typo: functional

> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> ---
>  arch/arm64/kvm/mmio.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c
> index 3dd38a151d2a..fd5747279d27 100644
> --- a/arch/arm64/kvm/mmio.c
> +++ b/arch/arm64/kvm/mmio.c
> @@ -6,6 +6,7 @@
>  
>  #include <linux/kvm_host.h>
>  #include <asm/kvm_emulate.h>
> +#include <asm/kvm_mmu.h>
>  #include <trace/events/kvm.h>
>  
>  #include "trace.h"
> @@ -130,6 +131,10 @@ int io_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa)
>  	int len;
>  	u8 data_buf[8];
>  
> +	/* Check failed? Return to the guest for debriefing... */
> +	if (!kvm_check_ioguard_page(vcpu, fault_ipa))
> +		return 1;
> +
>  	/*
>  	 * No valid syndrome? Ask userspace for help if it has
>  	 * volunteered to do so, and bail out otherwise.
> @@ -156,6 +161,11 @@ int io_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa)
>  	len = kvm_vcpu_dabt_get_as(vcpu);
>  	rt = kvm_vcpu_dabt_get_rd(vcpu);
>  
> +	/* If we cross a page boundary, check that too... */
> +	if (((fault_ipa + len - 1) & PAGE_MASK) != (fault_ipa & PAGE_MASK) &&
> +	    !kvm_check_ioguard_page(vcpu, fault_ipa + len - 1))
> +		return 1;
> +

I find this a little odd as the checks straddle the invalid syndrome check,
meaning that the relative priorities of KVM_ARCH_FLAG_MMIO_GUARD and
KVM_ARCH_FLAG_RETURN_NISV_IO_ABORT_TO_USER are unclear.

Will