linux-kernel - Re: [RFC PATCH v5 038/104] KVM: x86/mmu: Allow per-VM override of the TDP max page level

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Date:   Fri, 1 Apr 2022 20:53:19 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     Kai Huang <kai.huang@...el.com>, isaku.yamahata@...el.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>, erdemaktas@...gle.com,
        Connor Kuehl <ckuehl@...hat.com>
Subject: Re: [RFC PATCH v5 038/104] KVM: x86/mmu: Allow per-VM override of
 the TDP max page level

On Fri, Apr 01, 2022, Isaku Yamahata wrote:
> On Fri, Apr 01, 2022 at 02:08:38PM +0000,
> Sean Christopherson <seanjc@...gle.com> wrote:
> 
> > On Fri, Apr 01, 2022, Kai Huang wrote:
> > > On Fri, 2022-03-04 at 11:48 -0800, isaku.yamahata@...el.com wrote:
> > > > From: Sean Christopherson <sean.j.christopherson@...el.com>
> > > > 
> > > > In the existing x86 KVM MMU code, there is already max_level member in
> > > > struct kvm_page_fault with KVM_MAX_HUGEPAGE_LEVEL initial value.  The KVM
> > > > page fault handler denies page size larger than max_level.
> > > > 
> > > > Add per-VM member to indicate the allowed maximum page size with
> > > > KVM_MAX_HUGEPAGE_LEVEL as default value and initialize max_level in struct
> > > > kvm_page_fault with it.
> > > > 
> > > > For the guest TD, the set per-VM value for allows maximum page size to 4K
> > > > page size.  Then only allowed page size is 4K.  It means large page is
> > > > disabled.
> > > 
> > > Do not support large page for TD is the reason that you want this change, but
> > > not the result.  Please refine a little bit.
> > 
> > Not supporting huge pages was fine for the PoC, but I'd prefer not to merge TDX
> > without support for huge pages.  Has any work been put into enabling huge pages?
> > If so, what's the technical blocker?  If not...
> 
> I wanted to get feedback on the approach (always set SPTE to REMOVED_SPTE,
> callback, set the SPTE to the final value instead of relying atomic update SPTE)
> before going further for large page.

Pretty please with a cherry on top, send an email calling out which areas and
patches you'd like "immediate" feedback on.  Putting that information in the cover
letter would have been extremely helpful.  I realize it's hard to balance providing
context for folks who don't know TDX with "instructions" for reviewers, but one of
the most helpful things you can do for reviewers is to make it explicitly clear
what _your_ expectations and wants are, _why_ you posted the series.   Usually that
information is implied, i.e. you want your patches merged, but that's obviously not
the case here.