linux-kernel - Re: [PATCH] x86/vmware: use unsigned integer for shifting

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Message-ID: <c3219731-5f7e-80ea-b193-f8c93d7c1bdc@csail.mit.edu>
Date:   Fri, 20 May 2022 13:29:34 +0200
From:   "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>
To:     Borislav Petkov <bp@...en8.de>,
        Shreenidhi Shedi <yesshedi@...il.com>
Cc:     amakhalov@...are.com, tglx@...utronix.de, mingo@...hat.com,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        virtualization@...ts.linux-foundation.org, pv-drivers@...are.com,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Shreenidhi Shedi <sshedi@...are.com>
Subject: Re: [PATCH] x86/vmware: use unsigned integer for shifting

Hi Shreenidhi,

Thank you for the patch!

On 5/20/22 2:26 AM, Borislav Petkov wrote:
> On Fri, May 20, 2022 at 12:58:57PM +0530, Shreenidhi Shedi wrote:
>> Shifting signed 32-bit value by 31 bits is implementation-defined
>> behaviour. Using unsigned is better option for this.
>>

Can you also add a "Fixes:" tag with the commit that introduced the
issue? I believe it is 4cca6ea04d31 ("x86/apic: Allow x2apic without
IR on VMware platform").

>> Signed-off-by: Shreenidhi Shedi <sshedi@...are.com>
>> ---
>>  arch/x86/kernel/cpu/vmware.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/cpu/vmware.c b/arch/x86/kernel/cpu/vmware.c
>> index c04b933f48d3..b28f789d3c56 100644
>> --- a/arch/x86/kernel/cpu/vmware.c
>> +++ b/arch/x86/kernel/cpu/vmware.c
>> @@ -476,7 +476,7 @@ static bool __init vmware_legacy_x2apic_available(void)
>>  {
>>  	uint32_t eax, ebx, ecx, edx;
>>  	VMWARE_CMD(GETVCPU_INFO, eax, ebx, ecx, edx);
>> -	return (eax & (1 << VMWARE_CMD_VCPU_RESERVED)) == 0 &&
>> +	return (eax & (1U << VMWARE_CMD_VCPU_RESERVED)) == 0 &&
>>  	       (eax & (1 << VMWARE_CMD_LEGACY_X2APIC)) != 0;
> 
> Or you can use the BIT() macro and simplify this expression even more:
> 
>        return !(eax & BIT(VMWARE_CMD_VCPU_RESERVED)) &&
>                (eax & BIT(VMWARE_CMD_LEGACY_X2APIC));
> 
> 

That's better indeed.

Regards,
Srivatsa