Messages in this thread | | | From | Jürgen Groß <> | Subject | Re: [PATCH v5 11/12] x86/paravirt: switch functions with custom code to ALTERNATIVE | Date | Tue, 9 Mar 2021 07:21:35 +0100 |
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On 08.03.21 19:30, Borislav Petkov wrote: > On Mon, Mar 08, 2021 at 01:28:43PM +0100, Juergen Gross wrote: >> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h >> index 36cd71fa097f..04b3067f31b5 100644 >> --- a/arch/x86/include/asm/paravirt.h >> +++ b/arch/x86/include/asm/paravirt.h >> @@ -137,7 +137,8 @@ static inline void write_cr0(unsigned long x) >> >> static inline unsigned long read_cr2(void) >> { >> - return PVOP_CALLEE0(unsigned long, mmu.read_cr2); >> + return PVOP_ALT_CALLEE0(unsigned long, mmu.read_cr2, >> + "mov %%cr2, %%rax;", ~X86_FEATURE_XENPV); > > Just some cursory poking first - indepth review later. > > Do I see this correctly that the negated feature can be expressed with, to use > this example here: > > ALTERNATIVE_TERNARY(mmu.read_cr2, X86_FEATURE_XENPV, "", "mov %%cr2, %%rax;"); > > ?
No.
This would leave the Xen-pv case with a nop, while we need it to call mmu.read_cr2().
In the Xen-pv case there must be _no_ alternative patching in order to have the paravirt patching do its patching (indirect->direct call).
This is exactly the reason why I need to "not feature".
The only other solution I can think of would be a "split static_call" handling using ALTERNATIVE_TERNARY():
ALTERNATIVE_TERNARY(initial_static_call(mmu.read_cr2), X86_FEATURE_XENPV, final_static_call(mmu.read_cr2), "mov %%cr2, %%rax;");
with initial_static_call() doing an indirect call, while final_static_call() would do a direct call.
Not sure we really want that.
Juergen [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |