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Message-ID: <ZQef3zAEYjZMkn9k@yzhao56-desk.sh.intel.com>
Date: Mon, 18 Sep 2023 08:54:55 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>,
Huacai Chen <chenhuacai@...nel.org>,
Michael Ellerman <mpe@...erman.id.au>,
Anup Patel <anup@...infault.org>,
Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Paul Moore <paul@...l-moore.com>,
James Morris <jmorris@...ei.org>,
"Serge E. Hallyn" <serge@...lyn.com>, <kvm@...r.kernel.org>,
<linux-arm-kernel@...ts.infradead.org>, <kvmarm@...ts.linux.dev>,
<linux-mips@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
<kvm-riscv@...ts.infradead.org>, <linux-riscv@...ts.infradead.org>,
<linux-fsdevel@...r.kernel.org>, <linux-mm@...ck.org>,
<linux-security-module@...r.kernel.org>,
<linux-kernel@...r.kernel.org>,
Chao Peng <chao.p.peng@...ux.intel.com>,
Fuad Tabba <tabba@...gle.com>,
Jarkko Sakkinen <jarkko@...nel.org>,
Anish Moorthy <amoorthy@...gle.com>,
Yu Zhang <yu.c.zhang@...ux.intel.com>,
Isaku Yamahata <isaku.yamahata@...el.com>,
Xu Yilun <yilun.xu@...el.com>,
Vlastimil Babka <vbabka@...e.cz>,
Vishal Annapurve <vannapurve@...gle.com>,
Ackerley Tng <ackerleytng@...gle.com>,
Maciej Szmigiero <mail@...iej.szmigiero.name>,
David Hildenbrand <david@...hat.com>,
Quentin Perret <qperret@...gle.com>,
Michael Roth <michael.roth@....com>,
Wang <wei.w.wang@...el.com>,
"Liam Merwick" <liam.merwick@...cle.com>,
Isaku Yamahata <isaku.yamahata@...il.com>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH v12 18/33] KVM: x86/mmu: Handle page fault for
private memory
On Fri, Sep 15, 2023 at 07:26:16AM -0700, Sean Christopherson wrote:
> On Fri, Sep 15, 2023, Yan Zhao wrote:
> > On Wed, Sep 13, 2023 at 06:55:16PM -0700, Sean Christopherson wrote:
> > ....
> > > +static void kvm_mmu_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
> > > + struct kvm_page_fault *fault)
> > > +{
> > > + kvm_prepare_memory_fault_exit(vcpu, fault->gfn << PAGE_SHIFT,
> > > + PAGE_SIZE, fault->write, fault->exec,
> > > + fault->is_private);
> > > +}
> > > +
> > > +static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
> > > + struct kvm_page_fault *fault)
> > > +{
> > > + int max_order, r;
> > > +
> > > + if (!kvm_slot_can_be_private(fault->slot)) {
> > > + kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> > > + return -EFAULT;
> > > + }
> > > +
> > > + r = kvm_gmem_get_pfn(vcpu->kvm, fault->slot, fault->gfn, &fault->pfn,
> > > + &max_order);
> > > + if (r) {
> > > + kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> > > + return r;
> > > + }
> > > +
> > > + fault->max_level = min(kvm_max_level_for_order(max_order),
> > > + fault->max_level);
> > > + fault->map_writable = !(fault->slot->flags & KVM_MEM_READONLY);
> > > +
> > > + return RET_PF_CONTINUE;
> > > +}
> > > +
> > > static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > > {
> > > struct kvm_memory_slot *slot = fault->slot;
> > > @@ -4293,6 +4356,14 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> > > return RET_PF_EMULATE;
> > > }
> > >
> > > + if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> > In patch 21,
> > fault->is_private is set as:
> > ".is_private = kvm_mem_is_private(vcpu->kvm, cr2_or_gpa >> PAGE_SHIFT)",
> > then, the inequality here means memory attribute has been updated after
> > last check.
> > So, why an exit to user space for converting is required instead of a mere retry?
> >
> > Or, is it because how .is_private is assigned in patch 21 is subjected to change
> > in future?
>
> This. Retrying on SNP or TDX would hang the guest. I suppose we could special
Is this because if the guest access a page in private way (e.g. via
private key in TDX), the returned page must be a private page?
> case VMs where .is_private is derived from the memory attributes, but the
> SW_PROTECTED_VM type is primary a development vehicle at this point. I'd like to
> have it mimic SNP/TDX as much as possible; performance is a secondary concern.
Ok. But this mimic is somewhat confusing as it may be problematic in below scenario,
though sane guest should ensure no one is accessing a page before doing memory
conversion.
CPU 0 CPU 1
access GFN A in private way
fault->is_private=true
convert GFN A to shared
set memory attribute of A to shared
faultin, mismatch and exit
set memory attribute of A
to private
vCPU access GFN A in shared way
fault->is_private = true
faultin, match and map a private PFN B
vCPU accesses private PFN B in shared way
>
> E.g. userspace needs to be prepared for "spurious" exits due to races on SNP and
> TDX, which this can theoretically exercise. Though the window is quite small so
> I doubt that'll actually happen in practice; which of course also makes it less
> important to retry instead of exiting.
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