Re: [RFC 07/19] KVM: x86/mmu: Factor wrprot for nested PML out of make_spte

From: Paolo Bonzini <pbonzini@redhat.com>
To: Ben Gardon <bgardon@google.com>, Sean Christopherson <seanjc@google.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	Peter Xu <peterx@redhat.com>, Peter Shier <pshier@google.com>,
	David Matlack <dmatlack@google.com>,
	Mingwei Zhang <mizhang@google.com>,
	Yulei Zhang <yulei.kernel@gmail.com>,
	Wanpeng Li <kernellwp@gmail.com>,
	Xiao Guangrong <xiaoguangrong.eric@gmail.com>,
	Kai Huang <kai.huang@intel.com>,
	Keqian Zhu <zhukeqian1@huawei.com>,
	David Hildenbrand <david@redhat.com>
Subject: Re: [RFC 07/19] KVM: x86/mmu: Factor wrprot for nested PML out of make_spte
Date: Thu, 18 Nov 2021 19:04:36 +0100	[thread overview]
Message-ID: <c09eaa85-21c8-eb67-fe2f-9ea028e846f2@redhat.com> (raw)
In-Reply-To: <CANgfPd-f+VXQJnz-LPuiy+rTDkSdw3zjUfozaqzgb8n0rv9STA@mail.gmail.com>

On 11/18/21 18:43, Ben Gardon wrote:
>> Aha!  The dependency on @vcpu can be avoided without having to take a flag from
>> the caller.  The shadow page has everything we need.  The check is really "is this
>> a page for L2 EPT".  The kvm_x86_ops.cpu_dirty_log_size gets us the EPT part, and
>> kvm_mmu_page.guest_mode gets us the L2 part.
>
> Haha that's way cleaner than what I was doing! Seems like an obvious
> solution in retrospect. I'll include this in the next version of the
> series I send out unless Paolo beats me and just merges it directly.
> Happy to give this my reviewed-by.

Yeah, I am including the early cleanup parts because it makes no sense
to hold off on them; and Sean's patch qualifies as well.

I can't blame you for not remembering role.guest_mode.  Jim added it for
a decidedly niche reason:

     commit 1313cc2bd8f6568dd8801feef446afbe43e6d313
     Author: Jim Mattson <jmattson@google.com>
     Date:   Wed May 9 17:02:04 2018 -0400

     kvm: mmu: Add guest_mode to kvm_mmu_page_role

     L1 and L2 need to have disjoint mappings, so that L1's APIC access
     page (under VMX) can be omitted from L2's mappings.

     Signed-off-by: Jim Mattson <jmattson@google.com>
     Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

though it's actually gotten a lot more important than just that:

     commit 992edeaefed682511bd173dabd2f54b1ce5387df
     Author: Liran Alon <liran.alon@oracle.com>
     Date:   Wed Nov 20 14:24:52 2019 +0200

     KVM: nVMX: Assume TLB entries of L1 and L2 are tagged differently if L0 use EPT

     Since commit 1313cc2bd8f6 ("kvm: mmu: Add guest_mode to kvm_mmu_page_role"),
     guest_mode was added to mmu-role and therefore if L0 use EPT, it will
     always run L1 and L2 with different EPTP. i.e. EPTP01!=EPTP02.

     Because TLB entries are tagged with EP4TA, KVM can assume
     TLB entries populated while running L2 are tagged differently
     than TLB entries populated while running L1.

Paolo