Messages in this thread Patch in this message | | | Date | Tue, 8 Dec 2020 19:43:15 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH v2 07/12] x86: add new features for paravirt patching |
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On Fri, Nov 20, 2020 at 12:46:25PM +0100, Juergen Gross wrote: > diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h > index dad350d42ecf..ffa23c655412 100644 > --- a/arch/x86/include/asm/cpufeatures.h > +++ b/arch/x86/include/asm/cpufeatures.h > @@ -237,6 +237,9 @@ > #define X86_FEATURE_VMCALL ( 8*32+18) /* "" Hypervisor supports the VMCALL instruction */ > #define X86_FEATURE_VMW_VMMCALL ( 8*32+19) /* "" VMware prefers VMMCALL hypercall instruction */ > #define X86_FEATURE_SEV_ES ( 8*32+20) /* AMD Secure Encrypted Virtualization - Encrypted State */ > +#define X86_FEATURE_NOT_XENPV ( 8*32+21) /* "" Inverse of X86_FEATURE_XENPV */ > +#define X86_FEATURE_NO_PVUNLOCK ( 8*32+22) /* "" No PV unlock function */ > +#define X86_FEATURE_NO_VCPUPREEMPT ( 8*32+23) /* "" No PV vcpu_is_preempted function */
Ew, negative features. ;-\
/me goes forward and looks at usage sites:
+ ALTERNATIVE_2 "jmp *paravirt_iret(%rip);", \ + "jmp native_iret;", X86_FEATURE_NOT_XENPV, \ + "jmp xen_iret;", X86_FEATURE_XENPV
Can we make that:
ALTERNATIVE_TERNARY "jmp *paravirt_iret(%rip);", "jmp xen_iret;", X86_FEATURE_XENPV, "jmp native_iret;", X86_FEATURE_XENPV,
where the last two lines are supposed to mean
X86_FEATURE_XENPV ? "jmp xen_iret;" : "jmp native_iret;"
Now, in order to convey that logic to apply_alternatives(), you can do:
struct alt_instr { s32 instr_offset; /* original instruction */ s32 repl_offset; /* offset to replacement instruction */ u16 cpuid; /* cpuid bit set for replacement */ u8 instrlen; /* length of original instruction */ u8 replacementlen; /* length of new instruction */ u8 padlen; /* length of build-time padding */ u8 flags; /* patching flags */ <--- THIS } __packed;
and yes, we have had the flags thing in a lot of WIP diffs over the years but we've never come to actually needing it.
Anyway, then, apply_alternatives() will do:
if (flags & ALT_NOT_FEATURE)
or something like that - I'm bad at naming stuff - then it should patch only when the feature is NOT set and vice versa.
There in that
if (!boot_cpu_has(a->cpuid)) {
branch.
Hmm?
> /* Intel-defined CPU features, CPUID level 0x00000007:0 (EBX), word 9 */ > #define X86_FEATURE_FSGSBASE ( 9*32+ 0) /* RDFSBASE, WRFSBASE, RDGSBASE, WRGSBASE instructions*/ > diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c > index 2400ad62f330..f8f9700719cf 100644 > --- a/arch/x86/kernel/alternative.c > +++ b/arch/x86/kernel/alternative.c > @@ -593,6 +593,18 @@ int alternatives_text_reserved(void *start, void *end) > #endif /* CONFIG_SMP */ > > #ifdef CONFIG_PARAVIRT > +static void __init paravirt_set_cap(void) > +{ > + if (!boot_cpu_has(X86_FEATURE_XENPV)) > + setup_force_cpu_cap(X86_FEATURE_NOT_XENPV); > + > + if (pv_is_native_spin_unlock()) > + setup_force_cpu_cap(X86_FEATURE_NO_PVUNLOCK); > + > + if (pv_is_native_vcpu_is_preempted()) > + setup_force_cpu_cap(X86_FEATURE_NO_VCPUPREEMPT); > +} > + > void __init_or_module apply_paravirt(struct paravirt_patch_site *start, > struct paravirt_patch_site *end) > { > @@ -616,6 +628,8 @@ void __init_or_module apply_paravirt(struct paravirt_patch_site *start, > } > extern struct paravirt_patch_site __start_parainstructions[], > __stop_parainstructions[]; > +#else > +static void __init paravirt_set_cap(void) { } > #endif /* CONFIG_PARAVIRT */ > > /* > @@ -723,6 +737,18 @@ void __init alternative_instructions(void) > * patching. > */ > > + paravirt_set_cap();
Can that be called from somewhere in the Xen init path and not from here? Somewhere before check_bugs() gets called.
> + /* > + * First patch paravirt functions, such that we overwrite the indirect > + * call with the direct call. > + */ > + apply_paravirt(__parainstructions, __parainstructions_end); > + > + /* > + * Then patch alternatives, such that those paravirt calls that are in > + * alternatives can be overwritten by their immediate fragments. > + */ > apply_alternatives(__alt_instructions, __alt_instructions_end);
Can you give an example here pls why the paravirt patching needs to run first?
Thx.
-- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette
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