Re: [RFC PATCH 01/45] KVM: arm/arm64: add missing MMIO data write-back

Andre Przywara
2016-04-05T12:10:36Z

On 29/03/16 13:33, Christoffer Dall wrote:
> On Fri, Mar 25, 2016 at 02:04:24AM +0000, Andre Przywara wrote:
>> When the kernel was handling a guest MMIO access internally, we need
>> to copy the emulation result into the run->mmio structure in order
>> for the kvm_handle_mmio_return() function to pick it up and inject
>> the result back into the guest.
>> Currently the only user of kvm_io_bus for ARM is the VGIC, which did
>> this copying itself, so this was not causing issues so far.
>> But with upcoming kvm_io_bus users we need to do the copying here.
>>
>> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>> ---
>> arch/arm/kvm/mmio.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/arm/kvm/mmio.c b/arch/arm/kvm/mmio.c
>> index 0f6600f..d5c2727 100644
>> --- a/arch/arm/kvm/mmio.c
>> +++ b/arch/arm/kvm/mmio.c
>> @@ -206,7 +206,7 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
>> run->mmio.is_write = is_write;
>> run->mmio.phys_addr = fault_ipa;
>> run->mmio.len = len;
>> - if (is_write)
>> + if (is_write || !ret)
>> memcpy(run->mmio.data, data_buf, len);
>
> I had a really hard time understanding this, how about this instead:

Admittedly this is the shortest possible fix. I also had a rework closer
to your version, which also avoided copying the arguments in some cases,
but I thought a smaller diff would be more suitable, since this is
actually a fix.

Shall I add a comment here or post my version of the fix instead?

Cheers,
Andre.

> diff --git a/arch/arm/kvm/mmio.c b/arch/arm/kvm/mmio.c
> index 0f6600f..0158e9e 100644
> --- a/arch/arm/kvm/mmio.c
> +++ b/arch/arm/kvm/mmio.c
> @@ -87,11 +87,10 @@ static unsigned long mmio_read_buf(char *buf, unsigned int len)
>
> /**
> * kvm_handle_mmio_return -- Handle MMIO loads after user space emulation
> + * or in-kernel IO emulation
> + *
> * @vcpu: The VCPU pointer
> * @run: The VCPU run struct containing the mmio data
> - *
> - * This should only be called after returning from userspace for MMIO load
> - * emulation.
> */
> int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
> {
> @@ -206,18 +205,19 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
> run->mmio.is_write = is_write;
> run->mmio.phys_addr = fault_ipa;
> run->mmio.len = len;
> - if (is_write)
> - memcpy(run->mmio.data, data_buf, len);
>
> if (!ret) {
> /* We handled the access successfully in the kernel. */
> + if (!is_write)
> + memcpy(run->mmio.data, data_buf, len);
> vcpu->stat.mmio_exit_kernel++;
> kvm_handle_mmio_return(vcpu, run);
> return 1;
> - } else {
> - vcpu->stat.mmio_exit_user++;
> }
>
> + if (is_write)
> + memcpy(run->mmio.data, data_buf, len);
> + vcpu->stat.mmio_exit_user++;
> run->exit_reason = KVM_EXIT_MMIO;
> return 0;
> }
> diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
> index 00429b3..63e99cb 100644
> --- a/virt/kvm/arm/vgic.c
> +++ b/virt/kvm/arm/vgic.c
> @@ -850,12 +850,6 @@ static int vgic_handle_mmio_access(struct kvm_vcpu *vcpu,
> updated_state = false;
> }
> spin_unlock(&dist->lock);
> - run->mmio.is_write = is_write;
> - run->mmio.len = len;
> - run->mmio.phys_addr = addr;
> - memcpy(run->mmio.data, val, len);
> -
> - kvm_handle_mmio_return(vcpu, run);
>
> if (updated_state)
> vgic_kick_vcpus(vcpu->kvm);
>