linux-kernel - Re: [PATCH 58/61] KVM: x86/mmu: Configure max page level during hardware setup

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Message-ID: <20200225210149.GH9245@linux.intel.com>
Date:   Tue, 25 Feb 2020 13:01:50 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 58/61] KVM: x86/mmu: Configure max page level during
 hardware setup

On Tue, Feb 25, 2020 at 03:43:36PM +0100, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@...el.com> writes:
> 
> > Configure the max page level during hardware setup to avoid a retpoline
> > in the page fault handler.  Drop ->get_lpage_level() as the page fault
> > handler was the last user.
> > @@ -6064,11 +6064,6 @@ static void svm_set_supported_cpuid(struct kvm_cpuid_entry2 *entry)
> >  	}
> >  }
> >  
> > -static int svm_get_lpage_level(void)
> > -{
> > -	return PT_PDPE_LEVEL;
> > -}
> 
> I've probably missed something but before the change, get_lpage_level()
> on AMD was always returning PT_PDPE_LEVEL, but after the change and when
> NPT is disabled, we set max_page_level to either PT_PDPE_LEVEL (when
> boot_cpu_has(X86_FEATURE_GBPAGES)) or PT_DIRECTORY_LEVEL
> (otherwise). This sounds like a change) unless we think that
> boot_cpu_has(X86_FEATURE_GBPAGES) is always true on AMD.

It looks like a functional change, but isn't.  kvm_mmu_hugepage_adjust()
caps the page size used by KVM's MMU at the minimum of ->get_lpage_level()
and the host's mapping level.  Barring an egregious bug in the kernel MMU,
the host page tables will max out at PT_DIRECTORY_LEVEL (2mb) unless
boot_cpu_has(X86_FEATURE_GBPAGES) is true.

In other words, this is effectively a "documentation" change.  I'll figure
out a way to explain this in the changelog...

        max_level = min(max_level, kvm_x86_ops->get_lpage_level());
        for ( ; max_level > PT_PAGE_TABLE_LEVEL; max_level--) {
                linfo = lpage_info_slot(gfn, slot, max_level);
                if (!linfo->disallow_lpage)
                        break;
        }

        if (max_level == PT_PAGE_TABLE_LEVEL)
                return PT_PAGE_TABLE_LEVEL;

        level = host_pfn_mapping_level(vcpu, gfn, pfn, slot);
        if (level == PT_PAGE_TABLE_LEVEL)
                return level;

        level = min(level, max_level); <---------