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X-MS-Exchange-CrossTenant-AuthSource: BYAPR03MB3623.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 15 May 2023 09:47:49.4046 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 335836de-42ef-43a2-b145-348c2ee9ca5b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: szcKUbr2ANnS+InQSqh12gq7/dQ/XrPf3phKNfwwrknbGhOQjgMSWiP77JtaFxZPr7xN8qGmvsbxjEHw7dp08hEu6ziZEqnRl+ChxjgPwoM= X-MS-Exchange-Transport-CrossTenantHeadersStamped: SN7PR03MB7294 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15/05/2023 10:28 am, Peter Zijlstra wrote: > RESET_CALL_DEPTH is a pretty fat monster and blows up UNTRAIN_RET to > 20 bytes: > > 19: 48 c7 c0 80 00 00 00 mov $0x80,%rax > 20: 48 c1 e0 38 shl $0x38,%rax > 24: 65 48 89 04 25 00 00 00 00 mov %rax,%gs:0x0 29: R_X86_64_32S pcpu_hot+0x10 > > Shrink it by 4 bytes: > > 0:   31 c0                   xor    %eax,%eax > 2:   48 0f ba e8 3f          bts    $0x3f,%rax > 7:   65 48 89 04 25 00 00 00 00      mov    %rax,%gs:0x0 > > Shrink RESET_CALL_DEPTH_FROM_CALL by 5 bytes by only setting al, the > other bits are shifted out (the same could be done for > RESET_CALL_DEPTH, but the xor+bts sequence has less depencies due to > the zeroing). > > Suggested-by: Andrew.Cooper3@citrix.com Andrew Cooper > Signed-off-by: Peter Zijlstra (Intel) > --- > arch/x86/include/asm/nospec-branch.h | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > --- a/arch/x86/include/asm/nospec-branch.h > +++ b/arch/x86/include/asm/nospec-branch.h > @@ -84,12 +84,12 @@ > movq $-1, PER_CPU_VAR(pcpu_hot + X86_call_depth); > > #define RESET_CALL_DEPTH \ > - mov $0x80, %rax; \ > - shl $56, %rax; \ > + xor %eax, %eax; \ > + bts $59, %rax; \ $63 ? The disassembly looks correct. ~Andrew