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SubjectRe: [PATCH 0/5] [v3] fix illegal use of __pa() in KVM code
On Sun, Feb 24, 2013 at 04:28:58PM -0500, Peter Hurley wrote:
>
> On Tue, 2013-01-22 at 13:24 -0800, Dave Hansen wrote:
> > This series fixes a hard-to-debug early boot hang on 32-bit
> > NUMA systems. It adds coverage to the debugging code,
> > adds some helpers, and eventually fixes the original bug I
> > was hitting.
>
> Hi Dave,
>
> Now that the alloc_remap() has been/is being removed, is most/all of
> this being reverted?
>
> I ask because I was fixing a different bug in KVM's para-virt clock and
> saw part of this series (from [PATCH 5/5] fix kvm's use of __pa() on
> percpu areas):
>
> diff -puN arch/x86/kernel/kvmclock.c~fix-kvm-__pa-use-on-percpu-areas arch/x86/kernel/kvmclock.c
> --- linux-2.6.git/arch/x86/kernel/kvmclock.c~fix-kvm-__pa-use-on-percpu-areas 2013-01-22 13:17:16.428317508 -0800
> +++ linux-2.6.git-dave/arch/x86/kernel/kvmclock.c 2013-01-22 13:17:16.432317541 -0800
> @@ -162,8 +162,8 @@ int kvm_register_clock(char *txt)
> int low, high, ret;
> struct pvclock_vcpu_time_info *src = &hv_clock[cpu].pvti;
>
> - low = (int)__pa(src) | 1;
> - high = ((u64)__pa(src) >> 32);
> + low = (int)slow_virt_to_phys(src) | 1;
> + high = ((u64)slow_virt_to_phys(src) >> 32);
> ret = native_write_msr_safe(msr_kvm_system_time, low, high);
> printk(KERN_INFO "kvm-clock: cpu %d, msr %x:%x, %s\n",
> cpu, high, low, txt);
>
I think this hunk was converted by mistake since the code does not use
per cpu allocations, but since __pa() and slow_virt_to_phys() should
return the same thing here it does not matter much.

> which confused me because hv_clock is the __va of allocated physical
> memory, not a per-cpu variable.
>
> mem = memblock_alloc(size, PAGE_SIZE);
> if (!mem)
> return;
> hv_clock = __va(mem);
>
> So in short, my questions are:
> 1) is the slow_virt_to_phys() necessary anymore?
> 2) if yes, does it apply to the code above?
> 3) if yes, would you explain in more detail what the 32-bit NUMA mm is
> doing, esp. wrt. when __va(__pa) is not identical across all cpus?
>
> Regards,
> Peter Hurley
>

--
Gleb.


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