Re: [PATCH for-5.1] xhci: fix valid.max_access_size to access address registers

[Laurent Vivier]

On 21/07/2020 11:17, Philippe Mathieu-Daudé wrote:
> On 7/21/20 10:33 AM, Laurent Vivier wrote:
>> QEMU XHCI advertises AC64 (64-bit addressing) but doesn't allow
>> 64-bit mode access in "runtime" and "operational" MemoryRegionOps.
>>
>> Set the max_access_size based on sizeof(dma_addr_t) as AC64 is set.
>>
>> XHCI specs:
>> "If the xHC supports 64-bit addressing (AC64 = ‘1’), then software
>> should write 64-bit registers using only Qword accesses.  If a
>> system is incapable of issuing Qword accesses, then writes to the
>> 64-bit address fields shall be performed using 2 Dword accesses;
>> low Dword-first, high-Dword second.  If the xHC supports 32-bit
>> addressing (AC64 = ‘0’), then the high Dword of registers containing
>> 64-bit address fields are unused and software should write addresses
>> using only Dword accesses"
> 
> You only describe the WRITE path. Is the READ path similar?

The specs text comes from Alexey. So I don't know. But I don't see any
reason to not have 64bit read if we have 64bit write.

> 
>>
>> The problem has been detected with SLOF, as linux kernel always accesses
>> registers using 32-bit access even if AC64 is set and revealed by
>> 5d971f9e6725 ("memory: Revert "memory: accept mismatching sizes in 
>> memory_region_access_valid"")
>>
>> Suggested-by: Alexey Kardashevskiy <aik@au1.ibm.com>
>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>> ---
>>  hw/usb/hcd-xhci.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/hw/usb/hcd-xhci.c b/hw/usb/hcd-xhci.c
>> index b330e36fe6cc..67a18fe2b64c 100644
>> --- a/hw/usb/hcd-xhci.c
>> +++ b/hw/usb/hcd-xhci.c
>> @@ -3184,7 +3184,7 @@ static const MemoryRegionOps xhci_oper_ops = {
>>      .read = xhci_oper_read,
>>      .write = xhci_oper_write,
>>      .valid.min_access_size = 4,
>> -    .valid.max_access_size = 4,
>> +    .valid.max_access_size = sizeof(dma_addr_t),
>>      .endianness = DEVICE_LITTLE_ENDIAN,
>>  };
>>  
>> @@ -3200,7 +3200,7 @@ static const MemoryRegionOps xhci_runtime_ops = {
>>      .read = xhci_runtime_read,
>>      .write = xhci_runtime_write,
>>      .valid.min_access_size = 4,
>> -    .valid.max_access_size = 4,
>> +    .valid.max_access_size = sizeof(dma_addr_t),
>>      .endianness = DEVICE_LITTLE_ENDIAN,
>>  };
> 
> I wonder if we shouldn't check the access size now, something like:
> 
> bool xhci_check_access_size(void *opaque, hwaddr addr,
>                             unsigned size, bool is_write,
>                             MemTxAttrs attrs);
> {
>     XHCIState *xhci = opaque;
> 
>     /* FIXME only for is_write??? */
>     return xhci->ac64 || size == 4;

I don't think it's needed as AC64 (in fact a bit in HCCPARAMS) is set
only if sizeof(dma_addr_t) != 4...

but I'm checking source code, and dma_addr_t is always uint64_t.

I think it should rely instead on TARGET_PHYS_ADDR_SPACE_BITS.

But this check has been removed by David in:

59a70ccd3be2 ("usb-xhci: Use PCI DMA helper functions")

Thanks,
Laurent