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

Andre Przywara
2015-04-09T15:48:14Z

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?

Cheers,
Andre.