linux-kernel - Re: [PATCH v7 11/14] KVM: Register/unregister the guest private memory regions

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Message-ID: <Ytgw8HAsKTmZaubv@google.com>
Date:   Wed, 20 Jul 2022 16:44:32 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org, linux-kselftest@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v7 11/14] KVM: Register/unregister the guest private
 memory regions

On Wed, Jul 06, 2022, Chao Peng wrote:
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 230c8ff9659c..bb714c2a4b06 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -914,6 +914,35 @@ static int kvm_init_mmu_notifier(struct kvm *kvm)
>  
>  #endif /* CONFIG_MMU_NOTIFIER && KVM_ARCH_WANT_MMU_NOTIFIER */
>  
> +#ifdef CONFIG_HAVE_KVM_PRIVATE_MEM
> +#define KVM_MEM_ATTR_PRIVATE	0x0001
> +static int kvm_vm_ioctl_set_encrypted_region(struct kvm *kvm, unsigned int ioctl,
> +					     struct kvm_enc_region *region)
> +{
> +	unsigned long start, end;

As alluded to in a different reply, because this will track GPAs instead of HVAs,
the type needs to be "gpa_t", not "unsigned long".  Oh, actually, they need to
be gfn_t, since those are what gets shoved into the xarray.

> +	void *entry;
> +	int r;
> +
> +	if (region->size == 0 || region->addr + region->size < region->addr)
> +		return -EINVAL;
> +	if (region->addr & (PAGE_SIZE - 1) || region->size & (PAGE_SIZE - 1))
> +		return -EINVAL;
> +
> +	start = region->addr >> PAGE_SHIFT;
> +	end = (region->addr + region->size - 1) >> PAGE_SHIFT;
> +
> +	entry = ioctl == KVM_MEMORY_ENCRYPT_REG_REGION ?
> +				xa_mk_value(KVM_MEM_ATTR_PRIVATE) : NULL;
> +
> +	r = xa_err(xa_store_range(&kvm->mem_attr_array, start, end,
> +					entry, GFP_KERNEL_ACCOUNT));

IIUC, this series treats memory as shared by default.  I think we should invert
that and have KVM's ABI be that all guest memory as private by default, i.e.
require the guest to opt into sharing memory instead of opt out of sharing memory.

And then the xarray would track which regions are shared.

Regarding mem_attr_array, it probably makes sense to explicitly include what it's
tracking in the name, i.e. name it {private,shared}_mem_array depending on whether
it's used to track private vs. shared memory.  If we ever need to track metadata
beyond shared/private then we can tweak the name as needed, e.g. if hardware ever
supports secondary non-ephemeral encryption keys.