Re: [Qemu-devel] [RFC] ARM/ARM64: KVM: Implement KVM_FLUSH_DCACHE_GPA ioctl

[Laszlo Ersek <lersek@redhat.com>]

On 05/07/15 19:01, Paolo Bonzini wrote:
> 
> 
> On 07/05/2015 18:56, Jérémy Fanguède wrote:
>> USB devices fail with a timeout error, as if the communication between
>> the kernel and the devices fail at a certain point:
>> usb 1-1: device not accepting address 5, error -110
>> usb usb1-port1: unable to enumerate USB device

This is consistent with what I saw in my earlier testing.

>> e1000 fails when the userspace tries to use it, with these type of
>> kernel messages:
>> e1000 0000:00:02.0 eth0: Detected Tx Unit Hang
>>   Tx Queue             <0>
>>   TDH                  <d>
>>   TDT                  <d>
>>   next_to_use          <d>
>>   next_to_clean        <9>
>> buffer_info[next_to_clean]
>>   time_stamp           <ffff9311>
>>   next_to_watch        <a>
>>   jiffies              <ffff956a>
>>   next_to_watch.status <0>
> 
> Can you find out what memory attributes the guest is using for the
> memory---and if it's uncached, why?

For USB, see "drivers/usb/core/hcd-pci.c", function usb_hcd_pci_probe():
it uses ioremap_nocache().

On the "why", that ioremap_nocache() call can be tracked to

http://git.kernel.org/cgit/linux/kernel/git/tglx/history.git/commit/?id=a914dd8b

(Feb 2002), which predates the kernel's move to git. I guess
ioremap_nocache() is used simply because USB host controllers are
supposed to programmed like that.

And, from "arch/arm64/include/asm/io.h":

#define ioremap_nocache(addr, size)     __ioremap((addr), (size),
__pgprot(PROT_DEVICE_nGnRE))

Thanks
Laszlo