linux-kernel - Re: [RFC PATCH v5 037/104] KVM: x86/mmu: Allow non-zero init value for shadow PTE

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Message-ID: <6fb0889f-090d-740b-8ace-8b95b97ebf71@redhat.com>
Date:   Tue, 5 Apr 2022 16:10:25 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...il.com, Jim Mattson <jmattson@...gle.com>,
        erdemaktas@...gle.com, Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC PATCH v5 037/104] KVM: x86/mmu: Allow non-zero init value
 for shadow PTE

On 3/4/22 20:48, isaku.yamahata@...el.com wrote:
>   static void mmu_spte_clear_no_track(u64 *sptep)
>   {
> -	__update_clear_spte_fast(sptep, 0ull);
> +	__update_clear_spte_fast(sptep, shadow_init_value);
>   }
>   

Please WARN_ON_ONCE if shadow_init_value is nonzero, and then keep 0ull 
as the argument.

I have not thought much of the steps that are needed if were to flip 
both bit 0 and bit 63, so let's at least document that with a WARN.

Otherwise,

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>

Paolo