Messages in this thread | | | Subject | Re: [PATCH v2 04/12] x86/xen: drop USERGS_SYSRET64 paravirt call | From | Jürgen Groß <> | Date | Wed, 2 Dec 2020 15:48:21 +0100 |
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On 02.12.20 13:32, Borislav Petkov wrote: > On Fri, Nov 20, 2020 at 12:46:22PM +0100, Juergen Gross wrote: >> @@ -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 > > ? >
I wanted to avoid the additional NOPs for the bare metal case.
If you don't mind them I can do as you are suggesting.
Juergen [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |