Re: Patch "x86/vdso: Fix output operand size of RDPID" has been added to the 6.16-stable tree

On Sun, Oct 12, 2025 at 09:10:13AM -0700, H. Peter Anvin wrote:

Ok, that's just gas being stupid and overinterpreting the fuzzy language in the SDM, then. It would have been a very good thing to put in the commit or, even better, a comment.

The APM says:

"RDPID reads the value of TSC_AUX MSR used by the RDTSCP instruction into the specified destination register. Normal operand size prefixes do not apply and the update is either 32 bit or 64 bit based on the current mode."

so I interpret this as

dst_reg = MSR_TSC_AUX

which is a full u64 write. Not a sign-extended 32-bit thing.

Now if the machine does something else, I'm all ears. But we can verify that very easily...