linux-kernel - Re: [PATCH v10 2/9] KVM: Introduce per-page memory attributes

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Message-ID: <Y+vN4L3N6f3fFlgA@google.com>
Date:   Tue, 14 Feb 2023 10:07:28 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Isaku Yamahata <isaku.yamahata@...il.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-arch@...r.kernel.org,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.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>,
        Arnd Bergmann <arnd@...db.de>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Miaohe Lin <linmiaohe@...wei.com>, 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>, tabba@...gle.com,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        wei.w.wang@...el.com
Subject: Re: [PATCH v10 2/9] KVM: Introduce per-page memory attributes

On Mon, Feb 13, 2023, Isaku Yamahata wrote:
> On Fri, Feb 10, 2023 at 12:35:30AM +0000,
> Sean Christopherson <seanjc@...gle.com> wrote:
> 
> > On Wed, Feb 08, 2023, Isaku Yamahata wrote:
> > > On Fri, Dec 02, 2022 at 02:13:40PM +0800,
> > > Chao Peng <chao.p.peng@...ux.intel.com> wrote:
> > > 
> > > > +static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
> > > > +					   struct kvm_memory_attributes *attrs)
> > > > +{
> > > > +	gfn_t start, end;
> > > > +	unsigned long i;
> > > > +	void *entry;
> > > > +	u64 supported_attrs = kvm_supported_mem_attributes(kvm);
> > > > +
> > > > +	/* flags is currently not used. */
> > > > +	if (attrs->flags)
> > > > +		return -EINVAL;
> > > > +	if (attrs->attributes & ~supported_attrs)
> > > > +		return -EINVAL;
> > > > +	if (attrs->size == 0 || attrs->address + attrs->size < attrs->address)
> > > > +		return -EINVAL;
> > > > +	if (!PAGE_ALIGNED(attrs->address) || !PAGE_ALIGNED(attrs->size))
> > > > +		return -EINVAL;
> > > > +
> > > > +	start = attrs->address >> PAGE_SHIFT;
> > > > +	end = (attrs->address + attrs->size - 1 + PAGE_SIZE) >> PAGE_SHIFT;
> > > > +
> > > > +	entry = attrs->attributes ? xa_mk_value(attrs->attributes) : NULL;
> > > > +
> > > > +	mutex_lock(&kvm->lock);
> > > > +	for (i = start; i < end; i++)
> > > > +		if (xa_err(xa_store(&kvm->mem_attr_array, i, entry,
> > > > +				    GFP_KERNEL_ACCOUNT)))
> > > > +			break;
> > > > +	mutex_unlock(&kvm->lock);
> > > > +
> > > > +	attrs->address = i << PAGE_SHIFT;
> > > > +	attrs->size = (end - i) << PAGE_SHIFT;
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +#endif /* CONFIG_HAVE_KVM_MEMORY_ATTRIBUTES */
> > > > +
> > > 
> > > If memslot isn't private, it should return error if private attribute is set.
> > 
> > Why?  I'd rather keep the two things separate.  If we enforce this sort of thing
> > at KVM_SET_MEMORY_ATTRIBUTES, then we also have to enforce it at
> > KVM_SET_USER_MEMORY_REGION.
> 
> For device assignment via shared GPA, non-private memory slot needs to be
> allowed.

That doesn't say anything about why setting attributes needs to poke into the
memslot.  The page fault path already kicks out to userspace if there's a
discrepancy between the attributes and the memslot, why is that insufficient?