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: <20220401202847.GA560021@private.email.ne.jp>
Date:   Fri, 1 Apr 2022 13:28:47 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Kai Huang <kai.huang@...el.com>, 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 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.
-- 
Isaku Yamahata <isaku.yamahata@...il.com>