linux-kernel - Re: [PATCH 2/2] KVM: LAPIC: Set the TDCR settable bits

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Date:   Tue, 21 Jul 2020 12:51:42 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 2/2] KVM: LAPIC: Set the TDCR settable bits

Wanpeng Li <kernellwp@...il.com> writes:

> From: Wanpeng Li <wanpengli@...cent.com>
>
> Only bits 0, 1, and 3 are settable, others are reserved for APIC_TDCR. 
> Let's record the settable value in the virtual apic page.
>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/lapic.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 4ce2ddd..8f7a14d 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2068,7 +2068,7 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
>  	case APIC_TDCR: {
>  		uint32_t old_divisor = apic->divide_count;
>  
> -		kvm_lapic_set_reg(apic, APIC_TDCR, val);
> +		kvm_lapic_set_reg(apic, APIC_TDCR, val & 0xb);
>  		update_divide_count(apic);
>  		if (apic->divide_count != old_divisor &&
>  				apic->lapic_timer.period) {

AFAIU bit 2 should be 0 and other upper bits are reserved. Checking on
bare hardware,

# wrmsr 0x83e 0xb
# rdmsr 0x83e
b
# wrmsr 0x83e 0xc
wrmsr: CPU 0 cannot set MSR 0x0000083e to 0x000000000000000c
# rdmsr 0x83e
b

Shouldn't we fail the write in case (val & ~0xb) ?

-- 
Vitaly