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

Christoffer Dall
2015-04-09T13:30:42Z

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?

>
> if (updated_state)
> vgic_kick_vcpus(vcpu->kvm);
> --
> 1.7.9.5
>