Re: [RFC][PATCH 2/2] fix kvm's use of __pa() on percpu areas

[Gleb Natapov <gleb@redhat.com>]

On Thu, Nov 29, 2012 at 12:53:56AM +0000, Dave Hansen wrote:
> 
> In short, it is illegal to call __pa() on an address holding
> a percpu variable.  The times when this actually matters are
> pretty obscure (certain 32-bit NUMA systems), but it _does_
> happen.  It is important to keep KVM guests working on these
> systems because the real hardware is getting harder and
> harder to find.
> 
> This bug manifested first by me seeing a plain hang at boot
> after this message:
> 
> 	CPU 0 irqstacks, hard=f3018000 soft=f301a000
> 
> or, sometimes, it would actually make it out to the console:
> 
> [    0.000000] BUG: unable to handle kernel paging request at ffffffff
> 
> I eventually traced it down to the KVM async pagefault code.
> This can be worked around by disabling that code either at
> compile-time, or on the kernel command-line.
> 
> The kvm async pagefault code was injecting page faults in
> to the guest which the guest misinterpreted because its
> "reason" was not being properly sent from the host.
> 
> The guest passes a physical address of an per-cpu async page
> fault structure via an MSR to the host.  Since __pa() is
> broken on percpu data, the physical address it sent was
> bascially bogus and the host went scribbling on random data.
> The guest never saw the real reason for the page fault (it
> was injected by the host), assumed that the kernel had taken
> a _real_ page fault, and panic()'d.
> 
> Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
> ---
> 
>  linux-2.6.git-dave/arch/x86/kernel/kvm.c |    9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff -puN arch/x86/kernel/kvm.c~fix-kvm-__pa-use-on-percpu-areas arch/x86/kernel/kvm.c
> --- linux-2.6.git/arch/x86/kernel/kvm.c~fix-kvm-__pa-use-on-percpu-areas	2012-11-29 00:39:59.130213376 +0000
> +++ linux-2.6.git-dave/arch/x86/kernel/kvm.c	2012-11-29 00:51:55.428091802 +0000
> @@ -284,9 +284,9 @@ static void kvm_register_steal_time(void
>  
>  	memset(st, 0, sizeof(*st));
>  
> -	wrmsrl(MSR_KVM_STEAL_TIME, (__pa(st) | KVM_MSR_ENABLED));
> +	wrmsrl(MSR_KVM_STEAL_TIME, (slow_virt_to_phys(st) | KVM_MSR_ENABLED));
Where is this slow_virt_to_phys() coming from? I do not find it in
kvm.git.

>  	printk(KERN_INFO "kvm-stealtime: cpu %d, msr %lx\n",
> -		cpu, __pa(st));
> +		cpu, slow_virt_to_phys(st));
>  }
>  
>  static DEFINE_PER_CPU(unsigned long, kvm_apic_eoi) = KVM_PV_EOI_DISABLED;
> @@ -311,7 +311,7 @@ void __cpuinit kvm_guest_cpu_init(void)
>  		return;
>  
>  	if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF) && kvmapf) {
> -		u64 pa = __pa(&__get_cpu_var(apf_reason));
> +		u64 pa = slow_virt_to_phys(&__get_cpu_var(apf_reason));
>  
>  #ifdef CONFIG_PREEMPT
>  		pa |= KVM_ASYNC_PF_SEND_ALWAYS;
> @@ -327,7 +327,8 @@ void __cpuinit kvm_guest_cpu_init(void)
>  		/* Size alignment is implied but just to make it explicit. */
>  		BUILD_BUG_ON(__alignof__(kvm_apic_eoi) < 4);
>  		__get_cpu_var(kvm_apic_eoi) = 0;
> -		pa = __pa(&__get_cpu_var(kvm_apic_eoi)) | KVM_MSR_ENABLED;
> +		pa = slow_virt_to_phys(&__get_cpu_var(kvm_apic_eoi))
> +			| KVM_MSR_ENABLED;
>  		wrmsrl(MSR_KVM_PV_EOI_EN, pa);
>  	}
>  
> _

--
			Gleb.

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