On Thu, Jul 10, 2014 at 04:51:06PM -0500, Joel Schopp wrote: > > On 07/10/2014 04:02 PM, Joel Schopp wrote: > > On 07/10/2014 03:25 PM, Christoffer Dall wrote: > >> On Wed, Jul 09, 2014 at 11:17:04AM -0500, Joel Schopp wrote: > >>> The current calculation for VTTBR_BADDR_MASK masks only 39 bits and not > >>> all 40 bits. That last bit is important as some systems allocate > >>> from near the top of the available address space. > >>> > >>> This patch is necessary to run KVM on an aarch64 SOC I have been testing. > >>> > >>> Signed-off-by: Joel Schopp <joel.schopp@xxxxxxx> > >>> --- > >>> arch/arm64/include/asm/kvm_arm.h | 2 +- > >>> 1 file changed, 1 insertion(+), 1 deletion(-) > >>> > >>> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h > >>> index 3d69030..b39e93f 100644 > >>> --- a/arch/arm64/include/asm/kvm_arm.h > >>> +++ b/arch/arm64/include/asm/kvm_arm.h > >>> @@ -148,7 +148,7 @@ > >>> #endif > >>> > >>> #define VTTBR_BADDR_SHIFT (VTTBR_X - 1) > >>> -#define VTTBR_BADDR_MASK (((1LLU << (40 - VTTBR_X)) - 1) << VTTBR_BADDR_SHIFT) > >>> +#define VTTBR_BADDR_MASK (0xffffffffffLLU) /* bits 0-39 */ > >>> #define VTTBR_VMID_SHIFT (48LLU) > >>> #define VTTBR_VMID_MASK (0xffLLU << VTTBR_VMID_SHIFT) > >>> > >>> > >> While this is obviously fixing a bug, it doesn't feel like the right > >> short-term fix. I'll have to go back and read the definitions of x in > >> BADDR[47:x] for VTTBR_EL2 exactly again, but the intended use of > >> VTTBR_BADDR_MASK (and the only occurence of it in C-code) is to deal > >> with alignment of the allocated pgd. > > I think there is some confusion. Before VTTBR_BADDR_MASK always > > evaluated to 0x7fffffffffLLU, after the change it always evaluates to > > 0xffffffffffLLU > > > > Neither before nor after the patch is it dealing with alignment. Any > > bits it throws away (bits 40-47) are most significant not least significant. > > > > I could have rewritten the macro like: > > > > #define VTTBR_BADDR_MASK (((1LLU << (40 - VTTBR_X + 1)) - 1) << VTTBR_BADDR_SHIFT) > > > > to correct the bug but it's my opinion that the existing code is quite > > obfuscated which is how the bug happened in the first place. It seemed > > easier to just actually mask the bits in a straightforward and easy to > > understand manner. I even added a comment so nobody has to count the fs ;) > > > I hate to reply to my own email correcting myself. But you were > correct. I will fix and resend a v2. Thanks, -Christoffer -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
