From: Kai Huang <kai.huang@intel.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Michael Roth <michael.roth@amd.com>,
Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [PATCH 4/4] KVM: SVM: Disable SEV-ES support if MMIO caching is disable
Date: Mon, 01 Aug 2022 21:30:34 +1200 [thread overview]
Message-ID: <610ba489b8c0c03f7ac004049e10972bf01de927.camel@intel.com> (raw)
In-Reply-To: <YuP66QVxyeT4wd5H@google.com>
On Fri, 2022-07-29 at 15:21 +0000, Sean Christopherson wrote:
> On Fri, Jul 29, 2022, Kai Huang wrote:
> > On Thu, 2022-07-28 at 22:17 +0000, Sean Christopherson wrote:
> > > Disable SEV-ES if MMIO caching is disabled as SEV-ES relies on MMIO SPTEs
> > > generating #NPF(RSVD), which are reflected by the CPU into the guest as
> > > a #VC. With SEV-ES, the untrusted host, a.k.a. KVM, doesn't have access
> > > to the guest instruction stream or register state and so can't directly
> > > emulate in response to a #NPF on an emulated MMIO GPA. Disabling MMIO
> > > caching means guest accesses to emulated MMIO ranges cause #NPF(!PRESENT),
> > > and those flavors of #NPF cause automatic VM-Exits, not #VC.
> > >
> > > Fixes: b09763da4dd8 ("KVM: x86/mmu: Add module param to disable MMIO caching (for testing)")
> > > Reported-by: Michael Roth <michael.roth@amd.com>
> > > Cc: Tom Lendacky <thomas.lendacky@amd.com>
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > > ---
>
> ...
>
> > > + /*
> > > + * SEV-ES requires MMIO caching as KVM doesn't have access to the guest
> > > + * instruction stream, i.e. can't emulate in response to a #NPF and
> > > + * instead relies on #NPF(RSVD) being reflected into the guest as #VC
> > > + * (the guest can then do a #VMGEXIT to request MMIO emulation).
> > > + */
> > > + if (!enable_mmio_caching)
> > > + goto out;
> > > +
> > >
> >
> > I am not familiar with SEV, but looks it is similar to TDX -- they both causes
> > #VE to guest instead of faulting into KVM. And they both require explicit call
> > from guest to do MMIO.
> >
> > In this case, does existing MMIO caching logic still apply to them?
>
> Yes, because TDX/SEV-ES+ need to generate #VE/#VC on emulated MMIO so that legacy
> (or intentionally unenlightened) software in the guest doesn't simply hang/die on
> memory accesses to emulated MMIO (as opposed to direct TDVMCALL/#VMGEXIT).
>
> > Should we still treat SEV and TDX's MMIO handling as MMIO caching being
> > enabled? Or perhaps another variable?
>
> I don't think a separate variable is necesary. At its core, KVM is still caching
> MMIO GPAs via magic SPTE values. The fact that it's required for functionality
> doesn't make the name wrong.
OK.
>
> SEV-ES+ in particular doesn't have a strong guarantee that inducing #VC via #NPF(RSVD)
> is always possible. Theoretically, an SEV-ES+ capable CPU could ship with an effective
> MAXPHYADDR=51 (after reducing the raw MAXPHYADDR) and C-bit=51, in which case there are
> no resered PA bits and thus no reserved PTE bits at all. That's obviously unlikely to
> happen, but if it does come up, then disabling SEV-ES+ due to MMIO caching not being
> possible is the desired behavior, e.g. either the CPU configuration is bad or KVM is
> lacking support for a newfangled way to support emulated MMIO (in a future theoretical
> SEV-* product).
I bet AMD will see this (your) response and never ship such chips :)
--
Thanks,
-Kai
next prev parent reply other threads:[~2022-08-01 9:30 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-28 22:17 [PATCH 0/4] KVM: x86/mmu: MMIO caching bug fixes Sean Christopherson
2022-07-28 22:17 ` [PATCH 1/4] KVM: x86: Tag kvm_mmu_x86_module_init() with __init Sean Christopherson
2022-07-29 2:14 ` Kai Huang
2022-07-28 22:17 ` [PATCH 2/4] KVM: x86/mmu: Fully re-evaluate MMIO caching when SPTE masks change Sean Christopherson
2022-07-29 2:39 ` Kai Huang
2022-07-29 15:07 ` Sean Christopherson
2022-08-01 9:24 ` Kai Huang
2022-08-01 14:15 ` Sean Christopherson
2022-08-01 20:46 ` Kai Huang
2022-08-01 23:20 ` Sean Christopherson
2022-08-02 0:05 ` Kai Huang
2022-08-02 21:15 ` Sean Christopherson
2022-08-02 22:19 ` Kai Huang
2022-08-02 23:05 ` Sean Christopherson
2022-08-02 23:42 ` Kai Huang
2022-07-28 22:17 ` [PATCH 3/4] KVM: SVM: Adjust MMIO masks (for caching) before doing SEV(-ES) setup Sean Christopherson
2022-07-29 2:06 ` Kai Huang
2022-07-29 18:15 ` Sean Christopherson
2022-07-28 22:17 ` [PATCH 4/4] KVM: SVM: Disable SEV-ES support if MMIO caching is disable Sean Christopherson
2022-07-29 2:12 ` Kai Huang
2022-07-29 15:21 ` Sean Christopherson
2022-08-01 9:30 ` Kai Huang [this message]
2022-07-29 1:09 ` [PATCH 0/4] KVM: x86/mmu: MMIO caching bug fixes Michael Roth
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=610ba489b8c0c03f7ac004049e10972bf01de927.camel@intel.com \
--to=kai.huang@intel.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=michael.roth@amd.com \
--cc=pbonzini@redhat.com \
--cc=seanjc@google.com \
--cc=thomas.lendacky@amd.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).