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

From: [email-masked] (Christoffer Dall)
Date: Fri, 10 Apr 2015 11:15:49 +0200

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 <[email-masked]>
> >> ---
> >> 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.

-Christoffer