On Mon, Aug 24, 2020 at 10:54:33AM +0200, Joerg Roedel wrote: > From: Joerg Roedel <jroedel@xxxxxxx> > > Early exception handling will use rd/wrgsbase in paranoid_entry/exit. > Enable the feature to avoid #UD exceptions on boot APs. > > Signed-off-by: Joerg Roedel <jroedel@xxxxxxx> > Link: https://lore.kernel.org/r/20200724160336.5435-38-joro@xxxxxxxxxx > --- > arch/x86/kernel/head_64.S | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S > index 08412f308de3..4622940134a5 100644 > --- a/arch/x86/kernel/head_64.S > +++ b/arch/x86/kernel/head_64.S > @@ -153,6 +153,13 @@ SYM_CODE_START(secondary_startup_64) > orl $X86_CR4_LA57, %ecx > 1: > #endif > + > + ALTERNATIVE "jmp .Lstartup_write_cr4", "", X86_FEATURE_FSGSBASE > + > + /* Early exception handling uses FSGSBASE on APs */ > + orl $X86_CR4_FSGSBASE, %ecx How is this supposed to work? Alternatives haven't run that early yet and that piece of code looks like this: ffffffff81000067: eb 06 jmp ffffffff8100006f <secondary_startup_64+0x1f> ffffffff81000069: 81 c9 00 00 01 00 or $0x10000,%ecx ffffffff8100006f: 0f 22 e1 mov %rcx,%cr4 so we'll never set X86_CR4_FSGSBASE during early boot. Stopping a guest with gdb just before that shows the same thing: Dump of assembler code from 0x1000069 to 0x100007b: => 0x0000000001000069: eb 06 jmp 0x1000071 0x000000000100006b: 81 c9 00 00 01 00 or $0x10000,%ecx 0x0000000001000071: 0f 22 e1 mov %rcx,%cr4 0x0000000001000074: 48 03 05 95 ff 20 01 add 0x120ff95(%rip),%rax # 0x2210010 the unconditional JMP is there and it hasn't been patched out yet. If you really need to test CPUID flags, you need to do something similar to what verify_cpu does that early. And looking at that thing: * verify_cpu, returns the status of longmode and SSE in register %eax. * 0: Success 1: Failure you could return the FSGSBASE CPUID bit there too and act accordingly. Hmm. -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
