linux-kernel - Re: [RFC PATCH v12 18/33] KVM: x86/mmu: Handle page fault for private memory

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Message-ID: <ef36db9d-bb9c-e042-2617-830cf44602de@linux.intel.com>
Date:   Thu, 21 Sep 2023 13:51:34 +0800
From:   Binbin Wu <binbin.wu@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Yan Zhao <yan.y.zhao@...el.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 9/15/2023 10:26 PM, 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
> 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.
So when .is_private is derived from the memory attributes, and if I 
didn't miss
anything, there is no explicit conversion mechanism introduced yet so 
far, does
it mean for pure sw-protected VM (withouth SNP/TDX), the page fault will be
handled according to the memory attributes setup by host/user vmm, no 
implicit
conversion will be triggered, right?


>
> 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.