Messages in this thread | | | Date | Wed, 07 May 2014 19:11:58 +0300 | From | Nadav Amit <> | Subject | Re: [PATCH 4/5] KVM: x86: Wrong register masking in 64-bit mode |
| |
On 5/7/14, 5:50 PM, Bandan Das wrote: > Nadav Amit <namit@cs.technion.ac.il> writes: > >> 32-bit operations are zero extended in 64-bit mode. Currently, the code does >> not handle them correctly and keeps the high bits. In 16-bit mode, the high >> 32-bits are kept intact. >> >> In addition, although it is not well-documented, when address override prefix > > It would be helpful to have a pointer in the SDM especially for cases > that are not well-documented. > >> is used with REP-string instruction, RCX high half is zeroed even if ECX was >> zero on the first iteration (as if an assignment was performed to ECX). >> >> Signed-off-by: Nadav Amit <namit@cs.technion.ac.il> >>
First, as for the different masking/zero-extending behavior in different modes, I guess this behavior is documented in SDM Volume 1, Section 3.3.7: "Address Calculations in 64-Bit Mode". It appears (from experiments on bare-metal) that it also regards ESI/EDI/ECX on REP-prefix instructions and I presume it regards ESP on stack operations and ECX on loop operations. I will ensure it soon.
Second, the behavior of zero-extending RCX even if ECX is initially zero was experienced on native environment (Intel SandyBridge), and we found no erratum regarding it. I actually found no documentation that describes this behavior. The term "not well-documented" is probably misleading, and will be clarified on v2.
Thanks, Nadav
| |