Re: [PATCH v2 04/12] x86/xen: drop USERGS_SYSRET64 paravirt call
From: Borislav Petkov <bp@alien8.de>
Date: 2020-12-02 12:33:22
Also in:
lkml, xen-devel
On Fri, Nov 20, 2020 at 12:46:22PM +0100, Juergen Gross wrote:
quoted hunk ↗ jump to hunk
@@ -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
Why such a big ALTERNATIVE when you can simply do:
/*
* 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.
*/
ALTERNATIVE "", "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
movq RCX(%rsp), %rcx;
movq RIP(%rsp), %r11;
cmpq %rcx, %r11; /* SYSRET requires RCX == RIP */ \
jne swapgs_restore_regs_and_return_to_usermode
?
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization