linux-kernel - Re: [PATCH v5 0/9] Enable Sub-page Write Protection Support

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Date:   Fri, 11 Oct 2019 09:11:54 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        yu.c.zhang@...el.com, alazar@...defender.com
Subject: Re: [PATCH v5 0/9] Enable Sub-page Write Protection Support

On Fri, Oct 11, 2019 at 12:48 AM Yang Weijiang <weijiang.yang@...el.com> wrote:
>
> On Thu, Oct 10, 2019 at 02:42:51PM -0700, Jim Mattson wrote:
> > On Tue, Sep 17, 2019 at 1:52 AM Yang Weijiang <weijiang.yang@...el.com> wrote:
> > >
> > > EPT-Based Sub-Page write Protection(SPP)is a HW capability which allows
> > > Virtual Machine Monitor(VMM) to specify write-permission for guest
> > > physical memory at a sub-page(128 byte) granularity. When this
> > > capability is enabled, the CPU enforces write-access check for sub-pages
> > > within a 4KB page.
> > >
> > > The feature is targeted to provide fine-grained memory protection for
> > > usages such as device virtualization, memory check-point and VM
> > > introspection etc.
> > >
> > > SPP is active when the "sub-page write protection" (bit 23) is 1 in
> > > Secondary VM-Execution Controls. The feature is backed with a Sub-Page
> > > Permission Table(SPPT), SPPT is referenced via a 64-bit control field
> > > called Sub-Page Permission Table Pointer (SPPTP) which contains a
> > > 4K-aligned physical address.
> > >
> > > To enable SPP for certain physical page, the gfn should be first mapped
> > > to a 4KB entry, then set bit 61 of the corresponding EPT leaf entry.
> > > While HW walks EPT, if bit 61 is set, it traverses SPPT with the guset
> > > physical address to find out the sub-page permissions at the leaf entry.
> > > If the corresponding bit is set, write to sub-page is permitted,
> > > otherwise, SPP induced EPT violation is generated.
> >
> > How do you handle sub-page permissions for instructions emulated by kvm?
> How about checking if the gpa is SPP protected, if it is, inject some
> exception to guest?
The SPP semantics are well-defined. If a kvm-emulated instruction
tries to write to a sub-page that is write-protected, then an
SPP-induced EPT violation should be synthesized.