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SubjectRe: [PATCH 4/4] x86/xen: drop USERGS_SYSRET64 paravirt call
On Mon, Nov 16, 2020 at 7:23 AM Juergen Gross <jgross@suse.com> wrote:
>
> USERGS_SYSRET64 is used to return from a syscall via sysret, but
> a Xen PV guest will nevertheless use the iret hypercall, as there
> is no sysret PV hypercall defined.
>
> So instead of testing all the prerequisites for doing a sysret and
> then mangling the stack for Xen PV again for doing an iret just use
> the iret exit from the beginning.
>
> This can easily be done via an ALTERNATIVE like it is done for the
> sysenter compat case already.
>
> While at it remove to stale sysret32 remnants.
>
> Signed-off-by: Juergen Gross <jgross@suse.com>

Acked-by: Andy Lutomirski <luto@kernel.org>

FWIW, you've lost the VGCF_in_syscall optimization. Let me see if I
can give it back to you better.

> ---
> arch/x86/entry/entry_64.S | 22 +++++++++-------------
> arch/x86/include/asm/irqflags.h | 6 ------
> arch/x86/include/asm/paravirt.h | 5 -----
> arch/x86/include/asm/paravirt_types.h | 8 --------
> arch/x86/kernel/asm-offsets_64.c | 2 --
> arch/x86/kernel/paravirt.c | 5 +----
> arch/x86/kernel/paravirt_patch.c | 4 ----
> arch/x86/xen/enlighten_pv.c | 1 -
> arch/x86/xen/xen-asm.S | 20 --------------------
> arch/x86/xen/xen-ops.h | 2 --
> 10 files changed, 10 insertions(+), 65 deletions(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index a876204a73e0..df865eebd3d7 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -46,14 +46,6 @@
> .code64
> .section .entry.text, "ax"
>
> -#ifdef CONFIG_PARAVIRT_XXL
> -SYM_CODE_START(native_usergs_sysret64)
> - UNWIND_HINT_EMPTY
> - swapgs
> - sysretq
> -SYM_CODE_END(native_usergs_sysret64)
> -#endif /* CONFIG_PARAVIRT_XXL */
> -
> /*
> * 64-bit SYSCALL instruction entry. Up to 6 arguments in registers.
> *
> @@ -123,12 +115,15 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
> * Try to use SYSRET instead of IRET if we're returning to
> * a completely clean 64-bit userspace context. If we're not,
> * go to the slow exit path.
> + * In the Xen PV case we must use iret anyway.
> */
> - movq RCX(%rsp), %rcx
> - movq RIP(%rsp), %r11
>
> - cmpq %rcx, %r11 /* SYSRET requires RCX == RIP */
> - jne swapgs_restore_regs_and_return_to_usermode
> + ALTERNATIVE __stringify( \
> + movq RCX(%rsp), %rcx; \
> + movq RIP(%rsp), %r11; \
> + cmpq %rcx, %r11; /* SYSRET requires RCX == RIP */ \
> + jne swapgs_restore_regs_and_return_to_usermode), \
> + "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV

I'm not in love with this save-a-few-bytes stringify, but I can live with it.

--Andy

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