linux-kernel - Re: [PATCH 1/3] xen/pv: allow pmu msr accesses to cause GP

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Message-ID: <3f69b45d-d8c3-1e23-7fc9-87ffb436ad29@suse.com>
Date:   Mon, 26 Sep 2022 17:33:13 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] xen/pv: allow pmu msr accesses to cause GP

On 26.09.22 17:29, Jan Beulich wrote:
> On 26.09.2022 16:18, Juergen Gross wrote:
>> Today pmu_msr_read() and pmu_msr_write() fall back to the safe variants
>> of read/write MSR in case the MSR access isn't emulated via Xen. Allow
>> the caller to select the potentially faulting variant by passing NULL
>> for the error pointer.
> 
> Maybe make this "the sole caller" or some such? Because if there were
> multiple, they might easily disagree on what the best meaning of passing
> NULL is.

Okay.

> 
>> --- a/arch/x86/xen/pmu.c
>> +++ b/arch/x86/xen/pmu.c
>> @@ -293,22 +293,24 @@ static bool xen_amd_pmu_emulate(unsigned int msr, u64 *val, bool is_read)
>>   bool pmu_msr_read(unsigned int msr, uint64_t *val, int *err)
>>   {
>>   	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL) {
>> -		if (is_amd_pmu_msr(msr)) {
>> -			if (!xen_amd_pmu_emulate(msr, val, 1))
>> -				*val = native_read_msr_safe(msr, err);
>> -			return true;
>> +		if (!is_amd_pmu_msr(msr))
>> +			return false;
>> +		if (!xen_amd_pmu_emulate(msr, val, 1)) {
>> +			*val = err ? native_read_msr_safe(msr, err)
>> +				   : native_read_msr(msr);
>>   		}
>> +		return true;
> 
> Minor remark: Fold this and ...
> 
>>   	} else {
>>   		int type, index;
>>   
>> -		if (is_intel_pmu_msr(msr, &type, &index)) {
>> -			if (!xen_intel_pmu_emulate(msr, val, type, index, 1))
>> -				*val = native_read_msr_safe(msr, err);
>> -			return true;
>> +		if (!is_intel_pmu_msr(msr, &type, &index))
>> +			return false;
>> +		if (!xen_intel_pmu_emulate(msr, val, type, index, 1)) {
>> +			*val = err ? native_read_msr_safe(msr, err)
>> +				   : native_read_msr(msr);
>>   		}
>> +		return true;
> 
> ... this by moving them ...
> 
>>   	}
>> -
>> -	return false;
>>   }
> 
> ... above here? You might even de-duplicate the native_read_msr{,_safe}()
> invocations by moving them out of the if/else ...

Oh, nice idea!


Juergen


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