On Tue, Jan 29, 2019 at 03:20:16PM +0200, Jarkko Sakkinen wrote:

On Thu, Jan 24, 2019 at 07:43:30AM +1300, Linus Torvalds wrote:

quoted

On Thu, Jan 24, 2019 at 4:36 AM Jarkko Sakkinen
[off-list ref] wrote:

quoted

quoted

Is it just that this particular hardware always happened to trigger
the ERMS case (ie "rep movsb")?

This is the particular snippet in question:

memcpy_fromio(buf, priv->rsp, 6);
expected = be32_to_cpup((__be32 *) &buf[2]);
if (expected > count || expected < 6)
        return -EIO;

Ok, strange.

So what *used* to happen is that the memcpy_fromio() would just expand
as a "memcpy()", and in this case, gcc would then inline the memcpy().
In fact, gcc does it as a 4-byte access and a two-byte access from
what I can tell.

I verified, and it is exactly as you stated:

   0xffffffff814aaa33 <+51>:	mov    (%rax),%edx
   0xffffffff814aaa35 <+53>:	mov    %edx,0x0(%rbp)
   0xffffffff814aaa38 <+56>:	movzwl 0x4(%rax),%eax
   0xffffffff814aaa3c <+60>:	mov    %ax,0x4(%rbp)

And your new version does exactly the same thing to the first six bytes
(with different opcode, but the same memory access pattern).

I think I have found the root cause:

memcpy_fromio(&__rsp_pa, &priv->regs_t->ctrl_rsp_pa, 8);

This is from crb_map_io(). This should be read as quad word.

I'll change it to ioread64() and see what happens. I don't know why it
even has used memcpy_fromio() in the first place. I guess, when I first
implemented the driver, I used that for no logical reason, and it has
worked since up until now.

/Jarkko