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

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Date:   Tue, 22 Nov 2016 10:12:54 -0500 (EST)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Brijesh Singh <brijesh.singh@....com>, kvm@...r.kernel.org,
        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



----- Original Message -----
> 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
> Sent: Tuesday, November 22, 2016 3:25:52 PM
> 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).

Duh, right, see also assign_register in arch/x86/kvm/emulate.c.

Paolo

> 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;
> >> +	}
>