Re: [PATCH 3/3] x86/alternative: Patch a single alternative location only once

["Jürgen Groß" <jgross@suse.com>]

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On 30.09.25 09:26, Peter Zijlstra wrote:
> On Mon, Sep 29, 2025 at 01:29:47PM +0200, Juergen Gross wrote:
>> Instead of patching a single location potentially multiple times in
>> case of nested ALTERNATIVE()s, do the patching only after having
>> evaluated all alt_instr instances for that location.
>>
>> This has multiple advantages:
>>
>> - In case of replacing an indirect with a direct call using the
>>    ALT_FLAG_DIRECT_CALL flag, there is no longer the need to have that
>>    instance before any other instances at the same location (the
>>    original instruction is needed for finding the target of the direct
>>    call).
>>
>> - In case of nested ALTERNATIVE()s there is no intermediate replacement
>>    visible. This avoids any problems in case e.g. an interrupt is
>>    happening between the single instances and the patched location is
>>    used during handling the interrupt.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> V2:
>> - new patch
>> ---
>>   arch/x86/kernel/alternative.c | 27 +++++++++++++++++++--------
>>   1 file changed, 19 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
>> index 735cc017f2d3..ccf07131cd47 100644
>> --- a/arch/x86/kernel/alternative.c
>> +++ b/arch/x86/kernel/alternative.c
>> @@ -648,6 +648,8 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
>>   	u8 insn_buff[MAX_PATCH_LEN];
>>   	u8 *instr;
>>   	struct alt_instr *a, *b;
>> +	unsigned int instances = 0;
>> +	bool patched = false;
>>   
>>   	DPRINTK(ALT, "alt table %px, -> %px", start, end);
>>   
>> @@ -677,9 +679,13 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
>>   		 * padding for all alt_instr entries for this site (nested
>>   		 * alternatives result in consecutive entries).
>>   		 */
>> -		for (b = a+1; b < end && instr_va(b) == instr_va(a); b++) {
>> -			u8 len = max(a->instrlen, b->instrlen);
>> -			a->instrlen = b->instrlen = len;
>> +		if (!instances) {
>> +			for (b = a+1; b < end && instr_va(b) == instr_va(a); b++) {
>> +				u8 len = max(a->instrlen, b->instrlen);
>> +				a->instrlen = b->instrlen = len;
>> +			}
>> +			instances = b - a;
>> +			patched = false;
>>   		}
>>   
>>   		instr = instr_va(a);
>> @@ -692,14 +698,19 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
>>   		 * - feature not present but ALT_FLAG_NOT is set to mean,
>>   		 *   patch if feature is *NOT* present.
>>   		 */
>> -		if (!boot_cpu_has(a->cpuid) == !(a->flags & ALT_FLAG_NOT)) {
>> -			memcpy(insn_buff, instr, a->instrlen);
>> -			optimize_nops(instr, insn_buff, a->instrlen);
>> -		} else {
>> +		if (!boot_cpu_has(a->cpuid) != !(a->flags & ALT_FLAG_NOT)) {
>>   			apply_one_alternative(instr, insn_buff, a);
>> +			patched = true;
>>   		}
>>   
>> -		text_poke_early(instr, insn_buff, a->instrlen);
>> +		instances--;
>> +		if (!instances) {
>> +			if (!patched) {
>> +				memcpy(insn_buff, instr, a->instrlen);
>> +				optimize_nops(instr, insn_buff, a->instrlen);
>> +			}
>> +			text_poke_early(instr, insn_buff, a->instrlen);
>> +		}
>>   	}
>>   
>>   	kasan_enable_current();
> 
> I think you lost the optimize_nops() call for the patched case.
> 
> That is, note how apply_one_alternative() does 0x90 padding, but then
> you only do optimize_nops() when !patched.

The call of optimize_nops() is part of text_poke_apply_relocation() when
patching, like without my series.


Juergen

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