Re: [PATCH 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels

[Brian Gerst]

On Mon, Feb 5, 2018 at 1:29 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Andy Lutomirski <luto@xxxxxxxxxx> 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.

--
Brian Gerst