Messages in this thread | | | Date | Fri, 15 Jun 2007 01:27:44 +0200 | From | Luca <> | Subject | Re: [kvm-devel] [BUG] Oops with KVM-27 |
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On 6/15/07, Luca Tettamanti <kronos.it@gmail.com> 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 majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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