linux-kernel - Re: [PATCH v1 2/3] kvm: svm: Add kvm_fast_pio

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Date:   Tue, 22 Nov 2016 08:25:52 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>, <kvm@...r.kernel.org>
CC:     <rkrcmar@...hat.com>, <joro@...tes.org>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>, <mingo@...hat.com>,
        <hpa@...or.com>, <tglx@...utronix.de>, <bp@...e.de>
Subject: Re: [PATCH v1 2/3] kvm: svm: Add kvm_fast_pio_in support

On 11/21/2016 8:50 AM, Paolo Bonzini wrote:
> 
> 
> On 14/11/2016 23:15, Brijesh Singh wrote:
>> +	/* For size less than 4 we merge, else we zero extend */
>> +	val = (size < 4) ? kvm_register_read(vcpu, VCPU_REGS_RAX) : 0;
> 
> Are you sure it shouldn't always zero extend the high 32-bits?  So "val"
> should be declared as u32.
> 
> Paolo

It should only zero extend when dealing with a 32-bit operation. Any use
of 8 or 16 bit registers leaves the upper 56 or 48 bits as is (see
section 3.1.2.3 in http://support.amd.com/TechDocs/24592.pdf).

Thanks,
Tom

> 
>> +	ret = emulator_pio_in_emulated(&vcpu->arch.emulate_ctxt, size, port,
>> +				       &val, 1);
>> +	if (ret) {
>> +		kvm_register_write(vcpu, VCPU_REGS_RAX, val);
>> +		return ret;
>> +	}