linux-kernel - Re: [PATCH 0/5] [v3] fix illegal use of _

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Date:	Sun, 24 Feb 2013 16:28:58 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Gleb Natapov <gleb@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 0/5] [v3] fix illegal use of __pa() in KVM code


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);

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


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