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

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Date:   Wed, 20 Jul 2022 16:21:37 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Gupta, Pankaj" <pankaj.gupta@....com>
Cc:     Chao Peng <chao.p.peng@...ux.intel.com>, 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 20, 2022, Gupta, Pankaj wrote:
> 
> > > > > > +bool __weak kvm_arch_private_mem_supported(struct kvm *kvm)

Use kvm_arch_has_private_mem(), both because "has" makes it obvious this is checking
a flag of sorts, and to align with other helpers of this nature (and with
CONFIG_HAVE_KVM_PRIVATE_MEM).

  $ git grep kvm_arch | grep supported | wc -l
  0
  $ git grep kvm_arch | grep has | wc -l
  26

> > > > > > +#ifdef CONFIG_HAVE_KVM_PRIVATE_MEM
> > > > > > +	case KVM_MEMORY_ENCRYPT_REG_REGION:
> > > > > > +	case KVM_MEMORY_ENCRYPT_UNREG_REGION: {
> > > > > > +		struct kvm_enc_region region;
> > > > > > +
> > > > > > +		if (!kvm_arch_private_mem_supported(kvm))
> > > > > > +			goto arch_vm_ioctl;
> > > > > > +
> > > > > > +		r = -EFAULT;
> > > > > > +		if (copy_from_user(&region, argp, sizeof(region)))
> > > > > > +			goto out;
> > > > > > +
> > > > > > +		r = kvm_vm_ioctl_set_encrypted_region(kvm, ioctl, &region);
> > > > > 
> > > > > this is to store private region metadata not only the encrypted region?
> > > > 
> > > > Correct.
> > > 
> > > Sorry for not being clear, was suggesting name change of this function from:
> > > "kvm_vm_ioctl_set_encrypted_region" to "kvm_vm_ioctl_set_private_region"
> > 
> > Though I don't have strong reason to change it, I'm fine with this and
> 
> Yes, no strong reason, just thought "kvm_vm_ioctl_set_private_region" would
> depict the actual functionality :)
> 
> > this name matches the above kvm_arch_private_mem_supported perfectly.
> BTW could not understand this, how "kvm_vm_ioctl_set_encrypted_region"
> matches "kvm_arch_private_mem_supported"?

Chao is saying that kvm_vm_ioctl_set_private_region() pairs nicely with
kvm_arch_private_mem_supported(), not that the "encrypted" variant pairs nicely.

I also like using "private" instead of "encrypted", though we should probably
find a different verb than "set", because calling "set_private" when making the
region shared is confusing.  I'm struggling to come up with a good alternative
though.

kvm_vm_ioctl_set_memory_region() is already taken by KVM_SET_USER_MEMORY_REGION,
and that also means that anything with "memory_region" in the name is bound to be
confusing.

Hmm, and if we move away from "encrypted", it probably makes sense to pass in
addr+size instead of a kvm_enc_region.

Maybe this?

static int kvm_vm_ioctl_set_or_clear_mem_private(struct kvm *kvm, gpa_t gpa,
					         gpa_t size, bool set_private)

and then:

#ifdef CONFIG_HAVE_KVM_PRIVATE_MEM
	case KVM_MEMORY_ENCRYPT_REG_REGION:
	case KVM_MEMORY_ENCRYPT_UNREG_REGION: {
		bool set = ioctl == KVM_MEMORY_ENCRYPT_REG_REGION;
		struct kvm_enc_region region;

		if (!kvm_arch_private_mem_supported(kvm))
			goto arch_vm_ioctl;

		r = -EFAULT;
		if (copy_from_user(&region, argp, sizeof(region)))
			goto out;

		r = kvm_vm_ioctl_set_or_clear_mem_private(kvm, region.addr,
							  region.size, set);
		break;
	}
#endif

I don't love it, so if someone has a better idea...