Re: [PATCH v2 07/12] x86: add new features for paravirt patching

["Jürgen Groß" <jgross@suse.com>]

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On 08.12.20 19:43, Borislav Petkov wrote:
> 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. ;-\

Hey, I already suggested to use ~FEATURE for that purpose (see
https://lore.kernel.org/lkml/f105a63d-6b51-3afb-83e0-e899ea40813e@suse.com/ 
).

> 
> /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,

Would we really want to specify the feature twice?

I'd rather make the syntax:

ALTERNATIVE_TERNARY <initial-code> <feature> <code-for-feature-set>
                                              <code-for-feature-unset>

as this ...

> 
> where the last two lines are supposed to mean
> 
> 			    X86_FEATURE_XENPV ? "jmp xen_iret;" : "jmp native_iret;"

... would match perfectly to this interpretation.

> 
> 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;

Hmm, using flags is an alternative (pun intended :-) ).

> 
> 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?

Fine with me (I'd prefer my ALTERNATIVE_TERNARY syntax, though).

> 
>>   /* 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.

No, this is needed for non-Xen cases, too (especially pv spinlocks).

> 
>> +	/*
>> +	 * 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?

Okay.


Juergen

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