On Mon, Oct 6, 2025 at 1:10 PM Peter Zijlstra <[email protected]> wrote: > > On Mon, Oct 06, 2025 at 04:20:03PM +0530, Naman Jain wrote: > > > I am facing issues in this approach, after moving the assembly code to a > > separate file, using static calls, and making it noinstr. > > > > We need to make a call to STATIC_CALL_TRAMP_STR(hv_hypercall_pg + offset) in > > the assembly code. This offset is populated at run time in the driver, so I > > have to pass this offset to the assembly function via function parameters or > > a shared variable. This leaves noinstr section and results in below warning: > > > > [1]: vmlinux.o: warning: objtool: __mshv_vtl_return_call+0x4f: call to > > mshv_vtl_call_addr() leaves .noinstr.text section > > > > > > To fix this, one of the ways was to avoid making indirect calls. So I used > > EXPORT_STATIC_CALL to export the static call *trampoline and key* for the > > static call we created in C driver. Then I figured, we could simply call > > __SCT__<static_callname> in assembly code and it should work fine. But then > > it leads to this error in objtool. > > Easiest solution is to create a second static_call and have > hv_set_hypercall_pg() set that to +offset.
Yes, my idea was to add +offset directly in the static_call_update, as you sketched below. Sorry if that wasn't too clear. I didn't think of using a static call also for the base of the page, since that's not what the assembly code needs. And I think we agree that you absolutely don't want indirect calls, as that makes register usage much simpler. This way static calls end up killing multiple birds with a stone. > Also, what's actually in that hypercall page that is so magical and > can't just be an ALTERNATIVE() ? Do you mean an alternative for VMCALL vs. VMMCALL? If so, that's just not guaranteed to work: "An attempt to invoke a hypercall by any other means (for example, copying the code from the hypercall code page to an alternate location and executing it from there) might result in an undefined operation (#UD) exception" (or it might not). Paolo
