Subject: Re: [PATCH] vfio: Fix endianness handling for emulated BARs

From: Alexey Kardashevskiy <[email-masked]>
Date: Thu, 19 Jun 2014 11:50:32 +1000

On 06/19/2014 10:50 AM, Alexey Kardashevskiy wrote:
> On 06/19/2014 04:35 AM, Alex Williamson wrote:
>> On Wed, 2014-06-18 at 21:36 +1000, Alexey Kardashevskiy wrote:
>>> VFIO exposes BARs to user space as a byte stream so userspace can
>>> read it using pread()/pwrite(). Since this is a byte stream, VFIO should
>>> not do byte swapping and simply return values as it gets them from
>>> PCI device.
>>>
>>> Instead, the existing code assumes that byte stream in read/write is
>>> little-endian and it fixes endianness for values which it passes to
>>> ioreadXX/iowriteXX helpers. This works for little-endian as PCI is
>>> little endian and le32_to_cpu/... are stubs.
>>
>> vfio read32:
>>
>> val = cpu_to_le32(ioread32(io + off));
>>
>> Where the typical x86 case, ioread32 is:
>>
>> #define ioread32(addr)          readl(addr)
>>
>> and readl is:
>>
>> __le32_to_cpu(__raw_readl(addr));
>>
>> So we do canceling byte swaps, which are both nops on x86, and end up
>> returning device endian, which we assume is little endian.
>>
>> vfio write32 is similar:
>>
>> iowrite32(le32_to_cpu(val), io + off);
>>
>> The implicit cpu_to_le32 of iowrite32() and our explicit swap cancel
>> out, so input data is device endian, which is assumed little.
>>
>>> This also works for big endian but rather by an accident: it reads 4 bytes
>>> from the stream (@val is big endian), converts to CPU format (which should
>>> be big endian) as it was little endian (@val becomes actually little
>>> endian) and calls iowrite32() which does not do swapping on big endian
>>> system.
>>
>> Really?
>>
>> In arch/powerpc/kernel/iomap.c iowrite32() is just a wrapper around
>> writel(), which seems to use the generic implementation, which does
>> include a cpu_to_le32.
> 
> 
> Ouch, wrong comment. iowrite32() does swapping. My bad.
> 
> 
>>
>> I also see other big endian archs like parisc doing cpu_to_le32 on
>> iowrite32, so I don't think this statement is true.  I imagine it's
>> probably working for you because the swap cancel.
>>
>>> This removes byte swapping and makes use ioread32be/iowrite32be
>>> (and 16bit versions) on big-endian systems. The "be" helpers take
>>> native endian values and do swapping at the moment of writing to a PCI
>>> register using one of "store byte-reversed" instructions.
>>
>> So now you want iowrite32() on little endian and iowrite32be() on big
>> endian, the former does a cpu_to_le32 (which is a nop on little endian)
>> and the latter does a cpu_to_be32 (which is a nop on big endian)...
>> should we just be using __raw_writel() on both?
> 
> 
> We can do that too. The beauty of iowrite32be on ppc64 is that it does not
> swap and write separately, it is implemented via the "Store Word
> Byte-Reverse Indexed X-form" single instruction.
> 
> And some archs (do not know which ones) may add memory barriers in their
> implementations of ioread/iowrite. __raw_writel is too raw :)
> 
>>  There doesn't actually
>> seem to be any change in behavior here, it just eliminates back-to-back
>> byte swaps, which are a nop on x86, but not power, right?
> 
> Exactly. No dependency for QEMU.

How about that:
===

VFIO exposes BARs to user space as a byte stream so userspace can
read it using pread()/pwrite(). Since this is a byte stream, VFIO should
not do byte swapping and simply return values as it gets them from
PCI device.

Instead, the existing code assumes that byte stream in read/write is
little-endian and it fixes endianness for values which it passes to
ioreadXX/iowriteXX helpers in native format. The IO helpers do swapping
again. Since both byte swaps are nops on little-endian host, this works.

This also works for big endian but rather by an accident: it reads 4 bytes
from the stream (@val is big endian), converts to CPU format (which should
be big endian) as it was little endian (and @val becomes actually little
endian) and calls iowrite32() which does swapping on big endian
system again. So byte swap gets cancelled, __raw_writel() receives
a native value and then
*(volatile unsigned int __force *)PCI_FIX_ADDR(addr) = v;
just does the right thing.

This removes byte swaps and makes use of ioread32be/iowrite32be
(and 16bit versions) which do explicit byte swapping at the moment
of write to a PCI register. PPC64 uses a special "Store Word
Byte-Reverse Indexed X-form" instruction which does swap and store.
===

any better?

>>> Suggested-by: Benjamin Herrenschmidt <[email-masked]>
>>> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
>>> ---
>>>  drivers/vfio/pci/vfio_pci_rdwr.c | 20 ++++++++++++++++----
>>>  1 file changed, 16 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/vfio/pci/vfio_pci_rdwr.c b/drivers/vfio/pci/vfio_pci_rdwr.c
>>> index 210db24..f363b5a 100644
>>> --- a/drivers/vfio/pci/vfio_pci_rdwr.c
>>> +++ b/drivers/vfio/pci/vfio_pci_rdwr.c
>>> @@ -21,6 +21,18 @@
>>>  
>>>  #include "vfio_pci_private.h"
>>>  
>>> +#ifdef __BIG_ENDIAN__
>>> +#define ioread16_native		ioread16be
>>> +#define ioread32_native		ioread32be
>>> +#define iowrite16_native	iowrite16be
>>> +#define iowrite32_native	iowrite32be
>>> +#else
>>> +#define ioread16_native		ioread16
>>> +#define ioread32_native		ioread32
>>> +#define iowrite16_native	iowrite16
>>> +#define iowrite32_native	iowrite32
>>> +#endif
>>> +
>>>  /*
>>>   * Read or write from an __iomem region (MMIO or I/O port) with an excluded
>>>   * range which is inaccessible.  The excluded range drops writes and fills
>>> @@ -50,9 +62,9 @@ static ssize_t do_io_rw(void __iomem *io, char __user *buf,
>>>  				if (copy_from_user(&val, buf, 4))
>>>  					return -EFAULT;
>>>  
>>> -				iowrite32(le32_to_cpu(val), io + off);
>>> +				iowrite32_native(val, io + off);
>>>  			} else {
>>> -				val = cpu_to_le32(ioread32(io + off));
>>> +				val = ioread32_native(io + off);
>>>  
>>>  				if (copy_to_user(buf, &val, 4))
>>>  					return -EFAULT;
>>> @@ -66,9 +78,9 @@ static ssize_t do_io_rw(void __iomem *io, char __user *buf,
>>>  				if (copy_from_user(&val, buf, 2))
>>>  					return -EFAULT;
>>>  
>>> -				iowrite16(le16_to_cpu(val), io + off);
>>> +				iowrite16_native(val, io + off);
>>>  			} else {
>>> -				val = cpu_to_le16(ioread16(io + off));
>>> +				val = ioread16_native(io + off);
>>>  
>>>  				if (copy_to_user(buf, &val, 2))
>>>  					return -EFAULT;
>>
>>
>>
> 
> 

-- 
Alexey