Re: [Qemu-devel] SPARC iommu mapping

[Joerg Platte <jplatte@naasa.net>]

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Am Mittwoch, 5. April 2006 19:36 schrieb Blue Swirl:
Hi!

> I don't have any better documentation either, I just coded against what
> Proll and Linux expect. But the theory of operation is simple. Much like

OK, I thought I was just too stupid to find good documentation :-)

> how the MMU translates CPU's virtual addresses to physical addresses for
> memory, IOMMU translates device virtual memory accesses to physical
> addresses. The VA to PA entries are found in a simple page table.

That's what I read from qemu's iommu functions.

> In the case of not finding a valid translation entry, IOMMU can't fault the
> device like normal MMU can easily fault the CPU. I don't know what should
> happen then, probably put the address to AFAR register and raise some
> interrupt, while the device (for example Ethernet controller) waiting for
> the data suffers in limbo. I think it would be strange for an OS to rely on
> this, so I guess it's a bug somewhere else. My guess for the valid bit is
> that it's used in a real IOMMU to select the entries that will be loaded to
> its internal translation buffer.

Maybe. 

> The DMA controllers for both ESP and Lance are within the same page. This
> means that in Qemu, DMA controller register accesses for either of them go
> to just one of these. It just happens to work, but maybe this causes the
> problem. You could try to confirm this by enabling also DEBUG_LANCE and see
> if there is troublesome activity in the Lance direction near the bad
> accesses.

Hmmm, I don't use the network. Just disk access. But I'll check this.

> Can you provide a test case so that I could try it as well?

I'm using the two attached programs. writetest is used inside qemu to directly 
write to /dev/sda (with "writetest /dev/sda"). readtest can then be used 
outside qemu to check the written data ("readtest imagefile"). 

Additionally, I logged the translated virtual address as mentioned in my first 
mail. The kernel is 2.6.13. 

regards,
Jörg

[-- Attachment #2: readtest.c --]
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#include <stdio.h>


int main(int argc, char**argv) {
  FILE *fp;
  unsigned char buffer[4];
  unsigned int i,j;
  unsigned int doprint=1;
  unsigned int d=0;
  if ((fp=fopen(argv[1], "r"))) {
    for (i=0; i<10000000; i+=4) {
      if (fread(&buffer, 4, 1, fp)!=1) {
        printf("error writing at byte %d\n", i);
        break;
      }
      d=0;
      for(j=0;j<4; j++) {
        if (buffer[3-j]!=((i>>(j*8))&0xff)) {
          if (doprint) {
            printf("data differs at address %08x\n", i);
          }
          d=1;
          break;
        }
      }
      if (d) {
        doprint=0;
      } else {
        doprint=1;
      }
    }
    fclose(fp);
  }
}

[-- Attachment #3: writetest.c --]
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#include <stdio.h>


int main(int argc, char**argv) {
  FILE *fp;
  unsigned int i;
  if ((fp=fopen(argv[1], "w"))) {
    for (i=0; i<10000000; i+=4) {
      if (fwrite(&i, 4, 1, fp)!=1) {
        printf("error writing at byte %d\n", i);
        break;
      }
    }
    fclose(fp);
  }
}