Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1216537 > unrolled thread

Re: [GIT PULL] Early batch of KVM changes for 4.3 merge window

Back to article view | Back to linux.kernel

This discussion starts older than the indexed window; earlier articles aren't shown. The article labeled Started by below is the oldest one visible, not the original post.

Contents

  Re: [GIT PULL] Early batch of KVM changes for 4.3 merge window Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-01 02:50 +0200
    Re: [GIT PULL] Early batch of KVM changes for 4.3 merge window Xiao Guangrong <guangrong.xiao@linux.intel.com> - 2015-09-01 08:00 +0200
      Re: [GIT PULL] Early batch of KVM changes for 4.3 merge window Paolo Bonzini <pbonzini@redhat.com> - 2015-09-01 19:10 +0200
    Re: [GIT PULL] Early batch of KVM changes for 4.3 merge window Paolo Bonzini <pbonzini@redhat.com> - 2015-09-01 19:10 +0200
      Re: [GIT PULL] Early batch of KVM changes for 4.3 merge window Xiao Guangrong <guangrong.xiao@linux.intel.com> - 2015-09-02 00:50 +0200

#1216537 — Re: [GIT PULL] Early batch of KVM changes for 4.3 merge window

Hmm:

On Fri, Aug 14, 2015 at 4:57 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> Xiao Guangrong (9):
>       KVM: MMU: fully check zero bits for sptes

The above commit causes an annoying new compiler warning.

The warning is bogus ("variable 'leaf' possibly uninitialized"),
because the use of the variable is protected by the 'bool reserved'
flag, but gcc is apparently not smart enough to understand that.

Since bogus warnings cause people to possibly ignore the *real*
warnings, this should be fixed. Maybe the code should get rid of that
'reserved' flag, and instead initialize "leaf" to zero, and use that
as the flag instead (since zero isn't a valid level)? That would
actually avoid an extra variable, and would get rid of the warning.

Hmm?

                 Linus
--
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/

[toc] | [next] | [standalone]

#1216609

Linus, I am sorry for the annoyance.

On 09/01/2015 08:47 AM, Linus Torvalds wrote:
> Hmm:
>
> On Fri, Aug 14, 2015 at 4:57 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>> Xiao Guangrong (9):
>>        KVM: MMU: fully check zero bits for sptes
>
> The above commit causes an annoying new compiler warning.
>
> The warning is bogus ("variable 'leaf' possibly uninitialized"),
> because the use of the variable is protected by the 'bool reserved'
> flag, but gcc is apparently not smart enough to understand that.
>
> Since bogus warnings cause people to possibly ignore the *real*
> warnings, this should be fixed. Maybe the code should get rid of that
> 'reserved' flag, and instead initialize "leaf" to zero, and use that
> as the flag instead (since zero isn't a valid level)? That would
> actually avoid an extra variable, and would get rid of the warning.
>

The logic in that code is: if 'reserved' is true, print out the info in
spte[root - leaf]. I am afraid it's not good to use 'leaf' both for the
array index and reserved indicator. Or if i missed something please let
me know.

Actually i triggered this warning in my another box and posted a patch
to fix it which can be found at:
http://lkml.iu.edu/hypermail/linux/kernel/1508.2/02771.html
I guess Paolo is currently busy with KVM forum so the patch has not been
reviewed yet.

The patch simply initialized 'leaf' to the highest value to stop printing
the info, but as you noticed this is no real problem in the code just
stop GCC's complaint.

--
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/

[toc] | [prev] | [next] | [standalone]

#1216973

On 01/09/2015 07:45, Xiao Guangrong wrote:
> 
> 
> Actually i triggered this warning in my another box and posted a patch
> to fix it which can be found at:
> http://lkml.iu.edu/hypermail/linux/kernel/1508.2/02771.html
> I guess Paolo is currently busy with KVM forum so the patch has not been
> reviewed yet.

Currently I'm busy with the Dolomites, actually.  I'll send a fix
together with the PPC+ARM pull request.

Paolo
--
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/

[toc] | [prev] | [next] | [standalone]

#1216976

On 01/09/2015 02:47, Linus Torvalds wrote:
> Hmm:
> 
> On Fri, Aug 14, 2015 at 4:57 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>> Xiao Guangrong (9):
>>       KVM: MMU: fully check zero bits for sptes
> 
> The above commit causes an annoying new compiler warning.
> 
> The warning is bogus ("variable 'leaf' possibly uninitialized"),
> because the use of the variable is protected by the 'bool reserved'
> flag, but gcc is apparently not smart enough to understand that.

Unfortunately it doesn't reproduce on all compiler versions.

Something like this should do it:

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index fb16a8ea3dee..3c745f3abde8 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3309,13 +3309,13 @@ walk_shadow_page_get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
 
 	walk_shadow_page_lockless_begin(vcpu);
 
-	for (shadow_walk_init(&iterator, vcpu, addr), root = iterator.level;
+	for (shadow_walk_init(&iterator, vcpu, addr),
+		 leaf = root = iterator.level;
 	     shadow_walk_okay(&iterator);
 	     __shadow_walk_next(&iterator, spte)) {
-		leaf = iterator.level;
 		spte = mmu_spte_get_lockless(iterator.sptep);
 
-		sptes[leaf - 1] = spte;
+		sptes[--leaf] = spte;
 
 		if (!is_shadow_present_pte(spte))
 			break;
@@ -3329,7 +3329,7 @@ walk_shadow_page_get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
 	if (reserved) {
 		pr_err("%s: detect reserved bits on spte, addr 0x%llx, dump hierarchy:\n",
 		       __func__, addr);
-		while (root >= leaf) {
+		while (root > leaf) {
 			pr_err("------ spte 0x%llx level %d.\n",
 			       sptes[root - 1], root);
 			root--;


But honestly I haven't even compiled it yet.  Xiao, what do you think?

Paolo

> Since bogus warnings cause people to possibly ignore the *real*
> warnings, this should be fixed. Maybe the code should get rid of that
> 'reserved' flag, and instead initialize "leaf" to zero, and use that
> as the flag instead (since zero isn't a valid level)? That would
> actually avoid an extra variable, and would get rid of the warning.
> 
> Hmm?
> 
>                  Linus
> --
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
--
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/

[toc] | [prev] | [next] | [standalone]

#1217137

On 09/02/2015 01:03 AM, Paolo Bonzini wrote:

>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index fb16a8ea3dee..3c745f3abde8 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -3309,13 +3309,13 @@ walk_shadow_page_get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
>
>   	walk_shadow_page_lockless_begin(vcpu);
>
> -	for (shadow_walk_init(&iterator, vcpu, addr), root = iterator.level;
> +	for (shadow_walk_init(&iterator, vcpu, addr),
> +		 leaf = root = iterator.level;
>   	     shadow_walk_okay(&iterator);
>   	     __shadow_walk_next(&iterator, spte)) {
> -		leaf = iterator.level;
>   		spte = mmu_spte_get_lockless(iterator.sptep);
>
> -		sptes[leaf - 1] = spte;
> +		sptes[--leaf] = spte;
>
>   		if (!is_shadow_present_pte(spte))
>   			break;
> @@ -3329,7 +3329,7 @@ walk_shadow_page_get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
>   	if (reserved) {
>   		pr_err("%s: detect reserved bits on spte, addr 0x%llx, dump hierarchy:\n",
>   		       __func__, addr);
> -		while (root >= leaf) {
> +		while (root > leaf) {
>   			pr_err("------ spte 0x%llx level %d.\n",
>   			       sptes[root - 1], root);
>   			root--;
>
>
> But honestly I haven't even compiled it yet.  Xiao, what do you think?
>

It looks good to me!

--
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/

[toc] | [prev] | [standalone]

Back to top | Article view | linux.kernel

csiph-web