Re: [XenPPC] [xenppc-unstable] [POWERPC][XEN] Erratum: Must clear larx/stcx reservation on exception

[Segher Boessenkool <segher@xxxxxxxxxxxxxxxxxxx>]

> [POWERPC][XEN] Erratum: Must clear larx/stcx reservation on exception
>
> PowerPC 970 Erratum that an "OS should execute a stcx in
> the interrupt handler to clear the reservation."
Where do you see this erratum?  I can't find it in any 970
errata sheet.  Furthermore, it's not a CPU bug at all; every
PowerPC on the planet works this way, it's part of the
architecture.  See the "Interrupt Processing" chapter in Book III.

>      stw r0, UREGS_cr(r1)            /* save CR */
>      mfspr r0, SPRN_HSPRG1
>      std r0, UREGS_r13(r1)           /* save R13 from HSPRG1 */
> +
> +    /* Blow away any reservation according to 970 errata after  
> saving CR */
> +    stdcx. r1, 0, r1
I hope it's okay to blow away not only the reservation, but also
the data that's at the memory address in GPR1, because there's
nothing preventing this store-conditional from succeeding AFAICS
(if the CPU happens to have a reservation there -- yeah quite
unlikely, but is it impossible?)


Segher


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