Re: [Qemu-devel] page fault during ins

[Fabrice Bellard <fabrice@bellard.org>]

Very interesting remark. If it explains a bug in one OS (for example the 
win 2000 disk full issue !), then it will become high priority.

Another issue could be problems with 'movsw': the CPU could verify that 
it can write before doing the read to avoid restarting problems when 
talking to memory mapped devices...

Fabrice.

Piotras wrote:
> Hi!
> 
> The current implementation of ins (Input from Port to String) is 
> not-restartable. If page fault occurs during write, the port read 
> is redone and for some devices this may give unexpected results.
> 
> This is different from what real CPU does. From my tests with 
> Pentium II it seems that the CPU first makes sure that data can 
> be written, and then issue IO read. I'm attaching my test program 
> -- it may do harm to your system, so be very careful. The test 
> assumes that there is a CDROM connected as master to second 
> IDE controller (hdc).
> 
> 
> Regards,
> 
> Piotrek
> 
> 
> ------------------------------------------------------------------------
> 
> #include <sys/io.h>
> #include <sys/mman.h>
> #include <signal.h>
> #include <stdio.h>
> #include <stdint.h>
> 
> 
> uint16_t *data;
> 
> void segv()
> {
>     printf("SEGV\n");
>     data = mmap(data, 4096, PROT_READ | PROT_WRITE,
>                 MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
>     if (data == MAP_FAILED) {
>         exit(1);
>     }
> }
> 
> int main()
> {
>     int i;
>     unsigned char v;
>  
>     data = mmap(0, 4096, PROT_NONE,
>                 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>     if (data == MAP_FAILED) {
>         exit(1);
>     }
>     signal(SIGSEGV, segv);
> 
>     iopl(3);
> 
>     do {
>         v = inb(0x0177);
>     } while ((v & 0xc0) != 0x40);
>     outb(0x00, 0x0176);
>     outb(0xa1, 0x0177);
>     do {
>         v = inb(0x0177);
>     } while ((v & 0x08) != 0x08);
>     insw(0x0170, data, 256);
>     for(i = 0; i < 256; ++ i) {
>         printf("%04x%c", data[i], (i % 8 != 7)? ' ': '\n');
>     }
> }
> 
> 
> ------------------------------------------------------------------------
> 
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