linux-kernel - Re: [PATCH v5 11/12] x86/paravirt: switch functions with custom code to ALTERNATIVE

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Date:   Tue, 9 Mar 2021 07:21:35 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Deep Shah <sdeep@...are.com>,
        "VMware, Inc." <pv-drivers@...are.com>
Subject: Re: [PATCH v5 11/12] x86/paravirt: switch functions with custom code
 to ALTERNATIVE

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

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