linux-kernel - Re: [PATCH] KVM: LAPIC: ARBPRI is a reserved register for x2APIC

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Date:   Fri, 5 Jul 2019 15:37:41 +0300
From:   Liran Alon <liran.alon@...cle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: LAPIC: ARBPRI is a reserved register for x2APIC



> On 5 Jul 2019, at 15:14, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> kvm-unit-tests were adjusted to match bare metal behavior, but KVM
> itself was not doing what bare metal does; fix that.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
> arch/x86/kvm/lapic.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index d6ca5c4f29f1..2e4470f2685a 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1318,7 +1318,7 @@ int kvm_lapic_reg_read(struct kvm_lapic *apic, u32 offset, int len,
> 	unsigned char alignment = offset & 0xf;
> 	u32 result;
> 	/* this bitmask has a bit cleared for each reserved register */
> -	static const u64 rmask = 0x43ff01ffffffe70cULL;
> +	u64 rmask = 0x43ff01ffffffe70cULL;

Why not rename this to “used_bits_mask” and calculate it properly by macros?
It seems a lot nicer than having a pre-calculated magic.

-Liran

> 
> 	if ((alignment + len) > 4) {
> 		apic_debug("KVM_APIC_READ: alignment error %x %d\n",
> @@ -1326,6 +1326,10 @@ int kvm_lapic_reg_read(struct kvm_lapic *apic, u32 offset, int len,
> 		return 1;
> 	}
> 
> +	/* ARBPRI is also reserved on x2APIC */
> +	if (apic_x2apic_mode(apic))
> +		rmask &= ~(1 << (APIC_ARBPRI >> 4));
> +
> 	if (offset > 0x3f0 || !(rmask & (1ULL << (offset >> 4)))) {
> 		apic_debug("KVM_APIC_READ: read reserved register %x\n",
> 			   offset);
> -- 
> 1.8.3.1
>