Re: [PATCH] KVM: arm/arm64: avoid using kvm_run for in-kernel emulation

Andre Przywara
2015-04-10T13:09:43Z

Hi Christoffer,

On 10/04/15 10:15, Christoffer Dall wrote:
> On Thu, Apr 09, 2015 at 04:55:45PM +0100, Andre Przywara wrote:
>> Hej Christoffer,
>>
>> On 09/04/15 14:30, Christoffer Dall wrote:
>>> On Sat, Mar 28, 2015 at 01:48:20AM +0000, Andre Przywara wrote:
>>>> Our in-kernel VGIC emulation still uses struct kvm_run briefly before
>>>> writing back the emulation result into the guest register. Using a
>>>> userspace mapped data structure within the kernel sounds dodgy, also
>>>> we do some extra copying at the moment at the end of the VGIC
>>>> emulation code.
>>>> Replace the usage of struct kvm_run in favour of passing separate
>>>> parameters into kvm_handle_mmio_return (and rename the function on
>>>> the way) to optimise the VGIC emulation. The real userland MMIO code
>>>> path does not change much.
>>>>
>>>> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>>>> ---
>>>> Hi,
>>>>
>>>> this is an optimization of the VGIC code totally removing struct
>>>> kvm_run from the VGIC emulation. In my eyes it provides a nice
>>>> cleanup and is a logical consequence of the kvm_io_bus patches (on
>>>> which it goes on top). On the other hand it is optional and I didn't
>>>> want to merge it with the already quite large last patch 11.
>>>> Marc, I leave it up to you whether you take this as part of the
>>>> kvm_io_bus series or not.
>>>>
>>>> Cheers,
>>>> Andre.
>>>>
>>>> arch/arm/include/asm/kvm_mmio.h | 3 +-
>>>> arch/arm/kvm/arm.c | 6 ++--
>>>> arch/arm/kvm/mmio.c | 55 ++++++++++++++++++-------------------
>>>> arch/arm64/include/asm/kvm_mmio.h | 3 +-
>>>> virt/kvm/arm/vgic.c | 8 ++----
>>>> 5 files changed, 37 insertions(+), 38 deletions(-)
>>>>
>>>> diff --git a/arch/arm/include/asm/kvm_mmio.h b/arch/arm/include/asm/kvm_mmio.h
>>>> index d8e90c8..53461a6 100644
>>>> --- a/arch/arm/include/asm/kvm_mmio.h
>>>> +++ b/arch/arm/include/asm/kvm_mmio.h
>>>> @@ -28,7 +28,8 @@ struct kvm_decode {
>>>> bool sign_extend;
>>>> };
>>>>
>>>> -int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run);
>>>> +int kvm_writeback_mmio_data(struct kvm_vcpu *vcpu, unsigned int len,
>>>> + void *val, gpa_t phys_addr);
>>>> int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
>>>> phys_addr_t fault_ipa);
>>>>
>>>> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
>>>> index e98370c..b837aef 100644
>>>> --- a/arch/arm/kvm/arm.c
>>>> +++ b/arch/arm/kvm/arm.c
>>>> @@ -506,8 +506,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>>> if (ret)
>>>> return ret;
>>>>
>>>> - if (run->exit_reason == KVM_EXIT_MMIO) {
>>>> - ret = kvm_handle_mmio_return(vcpu, vcpu->run);
>>>> + if (run->exit_reason == KVM_EXIT_MMIO && !run->mmio.is_write) {
>>>> + ret = kvm_writeback_mmio_data(vcpu, run->mmio.len,
>>>> + run->mmio.data,
>>>> + run->mmio.phys_addr);
>>>> if (ret)
>>>> return ret;
>>>> }
>>>> diff --git a/arch/arm/kvm/mmio.c b/arch/arm/kvm/mmio.c
>>>> index 974b1c6..3c57f96 100644
>>>> --- a/arch/arm/kvm/mmio.c
>>>> +++ b/arch/arm/kvm/mmio.c
>>>> @@ -86,38 +86,36 @@ static unsigned long mmio_read_buf(char *buf, unsigned int len)
>>>> }
>>>>
>>>> /**
>>>> - * kvm_handle_mmio_return -- Handle MMIO loads after user space emulation
>>>> - * @vcpu: The VCPU pointer
>>>> - * @run: The VCPU run struct containing the mmio data
>>>> + * kvm_writeback_mmio_data -- Handle MMIO loads after user space emulation
>>>> + * @vcpu: The VCPU pointer
>>>> + * @len: The length in Bytes of the MMIO access
>>>> + * @data_ptr: Pointer to the data to be written back into the guest
>>>> + * @phys_addr: Physical address of the originating MMIO access
>>>> *
>>>> * This should only be called after returning from userspace for MMIO load
>>>> - * emulation.
>>>> + * emulation. phys_addr is only used for the tracepoint output.
>>>> */
>>>> -int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>>> +int kvm_writeback_mmio_data(struct kvm_vcpu *vcpu, unsigned int len,
>>>> + void *data_ptr, gpa_t phys_addr)
>>>> {
>>>> unsigned long data;
>>>> - unsigned int len;
>>>> int mask;
>>>>
>>>> - if (!run->mmio.is_write) {
>>>> - len = run->mmio.len;
>>>> - if (len > sizeof(unsigned long))
>>>> - return -EINVAL;
>>>> + if (len > sizeof(unsigned long))
>>>> + return -EINVAL;
>>>>
>>>> - data = mmio_read_buf(run->mmio.data, len);
>>>> + data = mmio_read_buf(data_ptr, len);
>>>>
>>>> - if (vcpu->arch.mmio_decode.sign_extend &&
>>>> - len < sizeof(unsigned long)) {
>>>> - mask = 1U << ((len * 8) - 1);
>>>> - data = (data ^ mask) - mask;
>>>> - }
>>>> -
>>>> - trace_kvm_mmio(KVM_TRACE_MMIO_READ, len, run->mmio.phys_addr,
>>>> - data);
>>>> - data = vcpu_data_host_to_guest(vcpu, data, len);
>>>> - *vcpu_reg(vcpu, vcpu->arch.mmio_decode.rt) = data;
>>>> + if (vcpu->arch.mmio_decode.sign_extend &&
>>>> + len < sizeof(unsigned long)) {
>>>> + mask = 1U << ((len * 8) - 1);
>>>> + data = (data ^ mask) - mask;
>>>> }
>>>>
>>>> + trace_kvm_mmio(KVM_TRACE_MMIO_READ, len, phys_addr, data);
>>>> + data = vcpu_data_host_to_guest(vcpu, data, len);
>>>> + *vcpu_reg(vcpu, vcpu->arch.mmio_decode.rt) = data;
>>>> +
>>>> return 0;
>>>> }
>>>>
>>>> @@ -201,18 +199,19 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
>>>> data_buf);
>>>> }
>>>>
>>>> - /* Now prepare kvm_run for the potential return to userland. */
>>>> - run->mmio.is_write = is_write;
>>>> - run->mmio.phys_addr = fault_ipa;
>>>> - run->mmio.len = len;
>>>> - memcpy(run->mmio.data, data_buf, len);
>>>> -
>>>> if (!ret) {
>>>> /* We handled the access successfully in the kernel. */
>>>> - kvm_handle_mmio_return(vcpu, run);
>>>> + if (!is_write)
>>>> + kvm_writeback_mmio_data(vcpu, len, data_buf, fault_ipa);
>>>> return 1;
>>>> }
>>>>
>>>> + /* Now prepare the kvm_run structure for the return to userland. */
>>>> + run->mmio.phys_addr = fault_ipa;
>>>> + run->mmio.len = len;
>>>> + run->mmio.is_write = is_write;
>>>> + memcpy(run->mmio.data, data_buf, len);
>>>> run->exit_reason = KVM_EXIT_MMIO;
>>>> +
>>>> return 0;
>>>> }
>>>> diff --git a/arch/arm64/include/asm/kvm_mmio.h b/arch/arm64/include/asm/kvm_mmio.h
>>>> index 889c908..3b83475 100644
>>>> --- a/arch/arm64/include/asm/kvm_mmio.h
>>>> +++ b/arch/arm64/include/asm/kvm_mmio.h
>>>> @@ -31,7 +31,8 @@ struct kvm_decode {
>>>> bool sign_extend;
>>>> };
>>>>
>>>> -int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run);
>>>> +int kvm_writeback_mmio_data(struct kvm_vcpu *vcpu, unsigned int len,
>>>> + void *val, gpa_t phys_addr);
>>>> int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
>>>> phys_addr_t fault_ipa);
>>>>
>>>> diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
>>>> index b70174e..4047fc0 100644
>>>> --- a/virt/kvm/arm/vgic.c
>>>> +++ b/virt/kvm/arm/vgic.c
>>>> @@ -803,7 +803,6 @@ static int vgic_handle_mmio_access(struct kvm_vcpu *vcpu,
>>>> struct vgic_dist *dist = &vcpu->kvm->arch.vgic;
>>>> struct vgic_io_device *iodev = container_of(this,
>>>> struct vgic_io_device, dev);
>>>> - struct kvm_run *run = vcpu->run;
>>>> const struct vgic_io_range *range;
>>>> struct kvm_exit_mmio mmio;
>>>> bool updated_state;
>>>> @@ -832,12 +831,9 @@ 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 (!is_write)
>>>> + kvm_writeback_mmio_data(vcpu, len, val, addr);
>>>
>>> why do we do a kvm_writeback_mmio_data() call here in addition to
>>> io_mem_abort() ? Are we not doing this twice? What am I missing?
>>
>> You are missing: ... nothing, we are indeed doing it twice. This looks
>> like an oversight when converting from using a data buffer to a pointer.
>>
>> So we can drop this here, and io_mem_abort() takes care (tested
>> quickly). Shall I add a comment or just remove those two lines?
>>
>> Are you OK with the rest of the patch?
>>
> Roughly ok, I think it would be nicer to not rename
> kvm_handle_mmio_return() but call that as is, and then introduce
> kvm_writeback_mmio_data() as a static function called only within mmio.c
> and do the check for !run->mmio.is_write in the kvm_handle_mmio_return()
> function before calling kvm_writeback_mmio_data().
>
> That would reduce the impact of this patch and keep the run-loop code
> slightly less cluttered.

Very neat! Thanks for that suggestion! I think that didn't work before
because of the former usage in vgic, but now it is indeed much nicer
this way.
Will send out the patch ASAP.

Cheers,
Andre.