Subject: [PATCH v3 06/10] arm: simplify MMIO dispatching

From: [email-masked] (Andre Przywara)
Date: Wed, 17 Jun 2015 14:49:59 +0100

Hi Marc,

On 06/17/2015 01:48 PM, Marc Zyngier wrote:
> On 17/06/15 12:21, Andre Przywara wrote:
>> Currently we separate any incoming MMIO request into one of the ARM
>> memory map regions and take care to spare the GIC.
>> It turns out that this is unnecessary, as we only have one special
>> region (the IO port area in the first 64 KByte). The MMIO rbtree
>> takes care about unhandled MMIO ranges, so we can simply drop all the
>> special range checking (except that for the IO range) in
>> kvm_cpu__emulate_mmio().
>> As the GIC is handled in the kernel, a GIC MMIO access should never
>> reach userland (and we don't know what to do with it anyway).
>> This lets us delete some more code and simplifies future extensions
>> (like expanding the GIC regions).
>> To be in line with the other architectures, move the now simpler
>> code into a header file.
>>
>> Signed-off-by: Andre Przywara <[email-masked]>
>> ---
>>  arm/include/arm-common/kvm-arch.h     | 12 ------------
>>  arm/include/arm-common/kvm-cpu-arch.h | 14 ++++++++++++--
>>  arm/kvm-cpu.c                         | 16 ----------------
>>  3 files changed, 12 insertions(+), 30 deletions(-)
>>
>> diff --git a/arm/include/arm-common/kvm-arch.h b/arm/include/arm-common/kvm-arch.h
>> index 082131d..90d6733 100644
>> --- a/arm/include/arm-common/kvm-arch.h
>> +++ b/arm/include/arm-common/kvm-arch.h
>> @@ -45,18 +45,6 @@ static inline bool arm_addr_in_ioport_region(u64 phys_addr)
>>  	return phys_addr >= KVM_IOPORT_AREA && phys_addr < limit;
>>  }
>>  
>> -static inline bool arm_addr_in_virtio_mmio_region(u64 phys_addr)
>> -{
>> -	u64 limit = KVM_VIRTIO_MMIO_AREA + ARM_VIRTIO_MMIO_SIZE;
>> -	return phys_addr >= KVM_VIRTIO_MMIO_AREA && phys_addr < limit;
>> -}
>> -
>> -static inline bool arm_addr_in_pci_region(u64 phys_addr)
>> -{
>> -	u64 limit = KVM_PCI_CFG_AREA + ARM_PCI_CFG_SIZE + ARM_PCI_MMIO_SIZE;
>> -	return phys_addr >= KVM_PCI_CFG_AREA && phys_addr < limit;
>> -}
>> -
>>  struct kvm_arch {
>>  	/*
>>  	 * We may have to align the guest memory for virtio, so keep the
>> diff --git a/arm/include/arm-common/kvm-cpu-arch.h b/arm/include/arm-common/kvm-cpu-arch.h
>> index 36c7872..329979a 100644
>> --- a/arm/include/arm-common/kvm-cpu-arch.h
>> +++ b/arm/include/arm-common/kvm-cpu-arch.h
>> @@ -44,8 +44,18 @@ static inline bool kvm_cpu__emulate_io(struct kvm_cpu *vcpu, u16 port, void *dat
>>  	return false;
>>  }
>>  
>> -bool kvm_cpu__emulate_mmio(struct kvm_cpu *vcpu, u64 phys_addr, u8 *data,
>> -			   u32 len, u8 is_write);
>> +static inline bool kvm_cpu__emulate_mmio(struct kvm_cpu *vcpu, u64 phys_addr,
>> +					 u8 *data, u32 len, u8 is_write)
>> +{
>> +	if (arm_addr_in_ioport_region(phys_addr)) {
>> +		int direction = is_write ? KVM_EXIT_IO_OUT : KVM_EXIT_IO_IN;
>> +		u16 port = (phys_addr - KVM_IOPORT_AREA) & USHRT_MAX;
>> +
>> +		return kvm__emulate_io(vcpu, port, data, direction, len, 1);
>> +	}
>> +
>> +	return kvm__emulate_mmio(vcpu, phys_addr, data, len, is_write);
>> +}
>>  
>>  unsigned long kvm_cpu__get_vcpu_mpidr(struct kvm_cpu *vcpu);
>>  
>> diff --git a/arm/kvm-cpu.c b/arm/kvm-cpu.c
>> index ab08815..7780251 100644
>> --- a/arm/kvm-cpu.c
>> +++ b/arm/kvm-cpu.c
>> @@ -139,22 +139,6 @@ bool kvm_cpu__handle_exit(struct kvm_cpu *vcpu)
>>  	return false;
>>  }
>>  
>> -bool kvm_cpu__emulate_mmio(struct kvm_cpu *vcpu, u64 phys_addr, u8 *data,
>> -			   u32 len, u8 is_write)
>> -{
>> -	if (arm_addr_in_virtio_mmio_region(phys_addr)) {
>> -		return kvm__emulate_mmio(vcpu, phys_addr, data, len, is_write);
>> -	} else if (arm_addr_in_ioport_region(phys_addr)) {
>> -		int direction = is_write ? KVM_EXIT_IO_OUT : KVM_EXIT_IO_IN;
>> -		u16 port = (phys_addr - KVM_IOPORT_AREA) & USHRT_MAX;
>> -		return kvm__emulate_io(vcpu, port, data, direction, len, 1);
>> -	} else if (arm_addr_in_pci_region(phys_addr)) {
>> -		return kvm__emulate_mmio(vcpu, phys_addr, data, len, is_write);
>> -	}
> 
> Can you explain why this arm_addr_in_pci_region(phys_addr) check has
> disappeared in your updated version on this function? It may be a non
> issue, but I'd very much like to understand.

If you look above the calls to kvm__emulate_mmio() are exactly the same
for the PCI and the virtio_mmio region, also as the areas are
non-overlapping the if branches can be reordered.
arm_addr_in_virtio_mmio_region() is true between 64k and (1GB - GIC),
while arm_addr_in_pci_region() gives true between 1GB and 2GB.

So this translates into: do kvm__emulate_io() for anything below 64K and
kvm__emulate_mmio() for everything else except for the GIC area,
admittedly in a quite convoluted way.

So my patch just removes the check for the GIC region and rewrites it to
match that description in the last sentence, with the rationale given in
the commit message.
Does that make sense?
If you desperately want some extra barfing for misguided GIC requests,
I'd rather introduce that to the "no match" code path in
kvm__emulate_mmio or register some dummy MMIO regions for the GIC with
panic() handlers.

Cheers,
Andre.