On Mon, Feb 5, 2018 at 7:48 PM, Ingo Molnar <[email protected]> wrote: > > * Brian Gerst <[email protected]> wrote: > >> On Mon, Feb 5, 2018 at 1:29 PM, Ingo Molnar <[email protected]> wrote: >> > >> > * Andy Lutomirski <[email protected]> wrote: >> > >> >> [...] Clearing R10 is mostly useless in the syscall path because we'll >> >> just >> >> unconditionally reload it in do_syscall_64(). >> > >> > AFAICS do_syscall_64() doesn't touch R10 at all. So how does it reload R10? >> > >> > In fact do_syscall_64() as a C function does not touch R10, R11, R12, R13, >> > R14, >> > R15 - it passes their values through. >> > >> > What am I missing? >> >> The syscall ABI uses R10 for the 4th argument instead of RCX, because >> RCX gets clobbered by the SYSCALL instruction for RIP. > > But we only reload the syscall-entry value of R10 it into RCX (4th C function > argument): > > regs->ax = sys_call_table[nr]( > regs->di, regs->si, regs->dx, > regs->r10, regs->r8, regs->r9); > > while RCX is a clobbered register, so in practice, while it will be briefly > present in do_syscall_64() and the high level syscall functions, the value in > RCX > will be cleared from RCX in the overwhelming majority of cases. > > But the real R10 will survive much longer, because it's only used in a very > small > minority of the C functions!
Yes, indeed, brain fart on my part.
