From: Sean Christopherson <seanjc@google.com>
To: David Matlack <dmatlack@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
vkuznets@redhat.com
Subject: Re: [PATCH 00/23] KVM: MMU: MMU role refactoring
Date: Mon, 7 Feb 2022 23:27:27 +0000 [thread overview]
Message-ID: <YgGq31edopd6RMts@google.com> (raw)
In-Reply-To: <YgGmgMMR0dBmjW86@google.com>
On Mon, Feb 07, 2022, David Matlack wrote:
> On Fri, Feb 04, 2022 at 06:56:55AM -0500, Paolo Bonzini wrote:
> > The TDP MMU has a performance regression compared to the legacy
> > MMU when CR0 changes often. This was reported for the grsecurity
> > kernel, which uses CR0.WP to implement kernel W^X. In that case,
> > each change to CR0.WP unloads the MMU and causes a lot of unnecessary
> > work. When running nested, this can even cause the L1 to hardly
> > make progress, as the L0 hypervisor it is overwhelmed by the amount
> > of MMU work that is needed.
> >
> > The root cause of the issue is that the "MMU role" in KVM is a mess
> > that mixes the CPU setup (CR0/CR4/EFER, SMM, guest mode, etc.)
> > and the shadow page table format. Whenever something is different
> > between the MMU and the CPU, it is stored as an extra field in struct
> > kvm_mmu---and for extra bonus complication, sometimes the same thing
> > is stored in both the role and an extra field.
> >
> > So, this is the "no functional change intended" part of the changes
> > required to fix the performance regression. It separates neatly
> > the shadow page table format ("MMU role") from the guest page table
> > format ("CPU role"), and removes the duplicate fields.
>
> What do you think about calling this the guest_role instead of cpu_role?
> There is a bit of a precedent for using "guest" instead of "cpu" already
> for this type of concept (e.g. guest_walker), and I find it more
> intuitive.
Haven't looked at the series yet, but I'd prefer not to use guest_role, it's
too similar to is_guest_mode() and kvm_mmu_role.guest_mode. E.g. we'd end up with
static union kvm_mmu_role kvm_calc_guest_role(struct kvm_vcpu *vcpu,
const struct kvm_mmu_role_regs *regs)
{
union kvm_mmu_role role = {0};
role.base.access = ACC_ALL;
role.base.smm = is_smm(vcpu);
role.base.guest_mode = is_guest_mode(vcpu);
role.base.direct = !____is_cr0_pg(regs);
...
}
and possibly
if (guest_role.guest_mode)
...
which would be quite messy. Maybe vcpu_role if cpu_role isn't intuitive?
next prev parent reply other threads:[~2022-02-08 1:05 UTC|newest]
Thread overview: 72+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-04 11:56 [PATCH 00/23] KVM: MMU: MMU role refactoring Paolo Bonzini
2022-02-04 11:56 ` [PATCH 01/23] KVM: MMU: pass uses_nx directly to reset_shadow_zero_bits_mask Paolo Bonzini
2022-02-04 17:59 ` David Matlack
2022-02-05 14:52 ` Paolo Bonzini
2022-02-07 16:09 ` Sean Christopherson
2022-02-07 21:50 ` David Matlack
2022-02-10 0:30 ` Sean Christopherson
2022-02-10 16:38 ` Paolo Bonzini
2022-02-04 11:56 ` [PATCH 02/23] KVM: MMU: nested EPT cannot be used in SMM Paolo Bonzini
2022-02-04 18:16 ` David Matlack
2022-02-09 22:43 ` Sean Christopherson
2022-02-04 11:56 ` [PATCH 03/23] KVM: MMU: remove valid from extended role Paolo Bonzini
2022-02-04 18:32 ` David Matlack
2022-02-05 14:50 ` Paolo Bonzini
2022-02-09 22:54 ` Sean Christopherson
2022-02-10 9:53 ` Paolo Bonzini
2022-02-04 11:56 ` [PATCH 04/23] KVM: MMU: constify uses of struct kvm_mmu_role_regs Paolo Bonzini
2022-02-04 18:41 ` David Matlack
2022-02-09 22:57 ` Sean Christopherson
2022-02-04 11:57 ` [PATCH 05/23] KVM: MMU: pull computation of kvm_mmu_role_regs to kvm_init_mmu Paolo Bonzini
2022-02-04 18:45 ` David Matlack
2022-02-04 11:57 ` [PATCH 06/23] KVM: MMU: load new PGD once nested two-dimensional paging is initialized Paolo Bonzini
2022-02-04 19:18 ` David Matlack
2022-02-07 13:50 ` Paolo Bonzini
2022-02-07 14:35 ` Paolo Bonzini
2022-02-09 12:34 ` Paolo Bonzini
2022-02-04 11:57 ` [PATCH 07/23] KVM: MMU: remove kvm_mmu_calc_root_page_role Paolo Bonzini
2022-02-04 19:32 ` David Matlack
2022-02-05 14:46 ` Paolo Bonzini
2022-02-10 0:47 ` Sean Christopherson
2022-02-10 9:52 ` Paolo Bonzini
2022-02-10 17:29 ` Sean Christopherson
2022-02-10 17:43 ` Paolo Bonzini
2022-02-04 11:57 ` [PATCH 08/23] KVM: MMU: rephrase unclear comment Paolo Bonzini
2022-02-04 19:38 ` David Matlack
2022-02-04 11:57 ` [PATCH 09/23] KVM: MMU: remove "bool base_only" arguments Paolo Bonzini
2022-02-04 19:41 ` David Matlack
2022-02-04 11:57 ` [PATCH 10/23] KVM: MMU: split cpu_role from mmu_role Paolo Bonzini
2022-02-04 21:57 ` David Matlack
2022-02-05 14:49 ` Paolo Bonzini
2022-02-07 21:38 ` David Matlack
2022-02-04 11:57 ` [PATCH 11/23] KVM: MMU: do not recompute root level from kvm_mmu_role_regs Paolo Bonzini
2022-02-07 22:10 ` David Matlack
2022-02-07 22:17 ` David Matlack
2022-02-04 11:57 ` [PATCH 12/23] KVM: MMU: remove ept_ad field Paolo Bonzini
2022-02-04 11:57 ` [PATCH 13/23] KVM: MMU: remove kvm_calc_shadow_root_page_role_common Paolo Bonzini
2022-02-07 22:25 ` David Matlack
2022-02-04 11:57 ` [PATCH 14/23] KVM: MMU: cleanup computation of MMU roles for two-dimensional paging Paolo Bonzini
2022-02-04 11:57 ` [PATCH 15/23] KVM: MMU: cleanup computation of MMU roles for shadow paging Paolo Bonzini
2022-02-04 11:57 ` [PATCH 16/23] KVM: MMU: remove extended bits from mmu_role Paolo Bonzini
2022-02-04 11:57 ` [PATCH 17/23] KVM: MMU: remove redundant bits from extended role Paolo Bonzini
2022-02-04 11:57 ` [PATCH 18/23] KVM: MMU: fetch shadow EFER.NX from MMU role Paolo Bonzini
2022-02-04 11:57 ` [PATCH 19/23] KVM: MMU: simplify and/or inline computation of shadow MMU roles Paolo Bonzini
2022-02-04 11:57 ` [PATCH 20/23] KVM: MMU: pull CPU role computation to kvm_init_mmu Paolo Bonzini
2022-02-07 22:42 ` David Matlack
2022-02-04 11:57 ` [PATCH 21/23] KVM: MMU: store shadow_root_level into mmu_role Paolo Bonzini
2022-02-07 23:00 ` David Matlack
2022-02-04 11:57 ` [PATCH 22/23] KVM: MMU: use cpu_role for root_level Paolo Bonzini
2022-02-07 23:01 ` David Matlack
2022-02-04 11:57 ` [PATCH 23/23] KVM: MMU: replace direct_map with mmu_role.direct Paolo Bonzini
2022-02-07 23:02 ` David Matlack
2022-02-07 23:08 ` [PATCH 00/23] KVM: MMU: MMU role refactoring David Matlack
2022-02-07 23:27 ` Sean Christopherson [this message]
2022-02-07 23:53 ` David Matlack
2022-02-10 1:11 ` Sean Christopherson
2022-02-10 11:58 ` Paolo Bonzini
2022-02-10 16:55 ` Sean Christopherson
2022-02-10 17:30 ` Paolo Bonzini
2022-02-10 19:28 ` Sean Christopherson
2022-02-09 22:31 ` Sean Christopherson
2022-02-10 9:54 ` Paolo Bonzini
2022-02-14 18:14 ` David Matlack
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