Re: Bus error question...

[Ralf Baechle <ralf@linux-mips.org>]

On Mon, May 02, 2005 at 01:55:08AM +0200, peter fuerst wrote:

> this question is posted here in the hope, it will be picked up and answered
> by some of the <*@*engr.sgi.com> gurus, i apologize to the other members of
> this mailing-list for annoying them with it as well ;-)

They've sold their souls to the evil empire.

> Is it save to assume, that memory bus errors (mc cpu_error_stat & 0x400) on
> IP28 - due to R10k's precise exception model - can be asynchronous only when
> caused by an aborted (misspeculated) instruction ?
> The R10k manual, experiences with spurious bus errors and experiments with
> "real" and speculated loads/stores seem to suggest this.
> Moreover, could it be enough to recognize the bus error as asynchrounous,
> when the exception code in cp0_cause doesn't say "Instruction bus error
> exception" (6) or "Data bus..." (7), but "Interrupt" (0) ?  (i.e. without
> analyzing the instruction at epc and register contents)
> 
> Rationale for this question: if a memory bus error can reliably be identified
> as originating from a misspeculated memory access, it would be possible to get
> rid of the myriads of cache barriers before *loads* (stores will remain
> protected by cache barriers anyway) again, and spending some thousand machine
> cycles on analyzing a bus error every three days of uptime is clearly more
> efficient than having a cache barrier in kernel code every seventeen
> instructions...

Supposedly cache barrier instructions on the R10000 are relativly cheap
but so far due to the lack of a need we haven't actually benchmarked that.

As I recall the issue loads would still fetch the line from memory
which in case of DMA buffers could result in stale data unless a cache
flush is being performed after the DMA as well.

  Ralf