On 2025-10-08 01:56, Jan Beulich wrote:
On 07.10.2025 21:38, Jason Andryuk wrote:
On 2025-10-07 08:22, Jan Beulich wrote:
On 30.09.2025 01:36, Jason Andryuk wrote:
On 2025-09-25 06:48, Jan Beulich wrote:
--- a/xen/arch/x86/include/asm/spec_ctrl.h
+++ b/xen/arch/x86/include/asm/spec_ctrl.h
@@ -73,7 +73,7 @@ static always_inline void spec_ctrl_new_
/* (ab)use alternative_input() to specify clobbers. */
alternative_input("", "DO_OVERWRITE_RSB xu=%=", X86_BUG_IBPB_NO_RET,
- : "rax", "rcx");
+ "i" (0) : "rax", "rcx");
"i" (0) is to work around the trailing comma in alternative_input() and
does nothing?
Yes. If more such "uses" appeared, we may want to introduce some kind of
abstraction.
Thanks for confirming.
Reviewed-by: Jason Andryuk <[email protected]>
Thanks.
Though I also wondered if just #define X86_BUG_MAX/X86_SYNTH_MAX
combined with a BUILD_BUG_ON might be good enough. Your approach avoids
the extra define but is more complicated. Anyway, just a thought.
How would that end up simplifying things? IOW what would the BUILD_BUG_ON()
look like that you're thinking about? After all X86_{SYNTH,BUG}_MAX aren't
meaningfully different from X86_NR_{SYNTH,BUG}.
Originally, I was thinking something like
XEN_CPUFEATURE(PDX_COMPRESSION, X86_SYNTH(31)) /* PDX compression */
+#define X86_SYNTH_MAX 31 /* Bump when adding flags */
and:
BUILD_BUG_ON( ((X86_SYNTH_MAX / 32) + 1) > X86_NR_SYNTH )
Not automated, but adding a new flag should make it obvious
X86_SYNTH_MAX should increase.
But as you point out the redundancy of X86_{SYNTH,BUG}_MAX and
X86_NR_{SYNTH,BUG}. But we could re-arrange to make X86_NR_{SYNTH,BUG}
calculated from X86_{SYNTH,BUG}_MAX like below.
Again, it's not automated, but it should make it harder to miss
increasing the value.
Regards,
Jason
diff --git i/xen/arch/x86/include/asm/cpufeatures.h
w/xen/arch/x86/include/asm/cpufeatures.h
index 0a98676c16..724eb1599f 100644
--- i/xen/arch/x86/include/asm/cpufeatures.h
+++ w/xen/arch/x86/include/asm/cpufeatures.h
@@ -7,7 +7,6 @@
#define FSCAPINTS FEATURESET_NR_ENTRIES
/* Synthetic words follow the featureset words. */
-#define X86_NR_SYNTH 1
#define X86_SYNTH(x) (FSCAPINTS * 32 + (x))
/* Synthetic features */
@@ -43,9 +42,10 @@ XEN_CPUFEATURE(IBPB_ENTRY_PV, X86_SYNTH(28)) /*
MSR_PRED_CMD used by Xen for
XEN_CPUFEATURE(IBPB_ENTRY_HVM, X86_SYNTH(29)) /* MSR_PRED_CMD used
by Xen for HVM */
XEN_CPUFEATURE(USE_VMCALL, X86_SYNTH(30)) /* Use VMCALL instead
of VMMCALL */
XEN_CPUFEATURE(PDX_COMPRESSION, X86_SYNTH(31)) /* PDX compression */
+#define X86_SYNTH_MAX 31 /* Bump when adding new flags. */
+#define X86_NR_SYNTH ((X86_SYNTH_MAX / 32) + 1)
/* Bug words follow the synthetic words. */
-#define X86_NR_BUG 1
#define X86_BUG(x) ((FSCAPINTS + X86_NR_SYNTH) * 32 + (x))
#define X86_BUG_FPU_PTRS X86_BUG( 0) /* (F)X{SAVE,RSTOR}
doesn't save/restore FOP/FIP/FDP. */
@@ -62,6 +62,8 @@ XEN_CPUFEATURE(PDX_COMPRESSION, X86_SYNTH(31)) /*
PDX compression */
#define X86_SPEC_BHB_TSX X86_BUG(19) /* Use clear_bhb_tsx for
BHI mitigation. */
#define X86_SPEC_BHB_LOOPS X86_BUG(20) /* Use clear_bhb_loops
for BHI mitigation.*/
#define X86_SPEC_BHB_LOOPS_LONG X86_BUG(21) /* Upgrade
clear_bhb_loops to the "long" sequence. */
+#define X86_BUX_MAX 21 /* Bump when adding new flags. */
+#define X86_NR_BUG ((X86_BUG_MAX / 32) + 1)
/* Total number of capability words, inc synth and bug words. */
#define NCAPINTS (FSCAPINTS + X86_NR_SYNTH + X86_NR_BUG) /* N 32-bit
words worth of info */
