On 6/15/07, Luca Tettamanti <[EMAIL PROTECTED]> wrote:
Il Fri, Jun 15, 2007 at 12:53:24AM +0200, Luca Tettamanti ha scritto: > Il Thu, Jun 14, 2007 at 11:26:29AM +0300, Avi Kivity ha scritto: > > Luca Tettamanti wrote: > > >With GOOD_APIC apic_read_around is a nop, while apic_write_around is a > > >normal write. With !GOOD_APIC apic_write_around writes to the APIC reg > > >using xchg. With !GOOD_APIC and this patch: > > > > > >--- include/asm-i386/apic.h~ 2007-04-26 05:08:32.000000000 +0200 > > >+++ include/asm-i386/apic.h 2007-06-13 22:35:00.000000000 +0200 > > >@@ -56,7 +56,8 @@ > > > static __inline fastcall void native_apic_write_atomic(unsigned long reg, > > > unsigned long v) > > > { > > >- xchg((volatile unsigned long *)(APIC_BASE+reg), v); > > >+// xchg((volatile unsigned long *)(APIC_BASE+reg), v); > > >+ *((volatile unsigned long *)(APIC_BASE+reg)) = v; > > > } > > > > > > static __inline fastcall unsigned long native_apic_read(unsigned long reg) > > > > > >The kernel boots fine. > > > > > > > Looking at the xchg emulation code, it seems fine, but clearly it > > isn't. > > Btw, I've put a printk in x86_emulate.c, where it prepares the operands > for the xchg operations: all the write_atomic are hitting this point, > so the write is lost somewhere in cmpxchg_emulated->write_emulated.Got it! The emulator skips the writeback if the old value is unchanged, so the apic doesn't see the write. Forcing the writeback: - if ((d & Mov) || (dst.orig_val != dst.val)) { - if ((d & Mov) || (dst.orig_val != dst.val) || isxchg) { seems to fix the issue :D I'm not sure that fix is correct though.
After a bit of thinking: it's correct but removes an optimization; furthermore it may miss other instructions that write to memory mapped areas. A more proper fix should be "force the writeback if dst.ptr is in some kind of mmio area". Ok, enough of reply-to-self. I'll go to sleep... Luca - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
