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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Message-ID: <YkeUDJP1AWKU/ixG@google.com>
Date:   Sat, 2 Apr 2022 00:08:44 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        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 Sat, Apr 02, 2022, Kai Huang wrote:
> On Fri, 2022-04-01 at 14:08 +0000, Sean Christopherson 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...
> 
> Hi Sean,
> 
> Is there any reason large page support must be included in the initial merge of
> TDX?  Large page is more about performance improvement I think.  Given this
> series is already very big, perhaps we can do it later.

I'm ok punting 1gb for now, but I want to have a high level of confidence that 2mb
pages will work without requiring significant churn in KVM on top of the initial
TDX support.  I suspect gaining that level of confidence will mean getting 95%+ of
the way to a fully working code base.  IIRC, 2mb wasn't expected to be terrible, it
was 1gb support where things started to get messy.