linux-kernel - Re: [PATCH v6 01/21] x86/tdx: Use enum to define page level of TDX supported page sizes

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Date:   Fri, 28 Oct 2022 00:47:52 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kirill@...temov.name" <kirill@...temov.name>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Li, Xiaoyao" <xiaoyao.li@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v6 01/21] x86/tdx: Use enum to define page level of TDX
 supported page sizes

On Thu, 2022-10-27 at 16:51 +0300, Kirill A. Shutemov wrote:
> On Thu, Oct 27, 2022 at 08:42:16AM +0000, Huang, Kai wrote:
> > On Thu, 2022-10-27 at 15:08 +0800, Li, Xiaoyao wrote:
> > > > @@ -663,27 +662,16 @@ static bool try_accept_one(phys_addr_t *start,
> > > > unsigned long len,
> > > >    	if (len < accept_size)
> > > >    		return false;
> > > >    
> > > > +	/* TDX only supports 4K/2M/1G page sizes */
> > > 
> > > yes, a page can be mapped as 1G size to TD via secure/shared EPT. But 
> > > for this particular TDX_ACCEPT_PAGE case, it only supports 4K and 2M 
> > > currently, which is defined in TDX module spec.
> > 
> > I checked the TDX module public spec, and it appears you are right.  But I am
> > not sure whether it will be changed in the future?
> 
> The spec says:
> 
> 	Level of the Secure EPT entry that maps the private page to be
> 	accepted: either 0 (4KB) or 1 (2MB) – see 20.5.1
> 
> Yes, it is about 4k and 2M, but it also refers 20.5.1, which lists sizes
> up to 16PB.

Also, I think we are mixing two things: 1) leaf page sizes  (4K/2M/1G);  2) page
table levels.  The latter has more levels than the former.  For try_accept_one()
(and TDX host code), we actually care only about the former.

KVM needs the latter (or both), so it's better for KVM to handle on its own.

> 
> Ultimately, it does not matter: if TDX module doesn't support the size or
> cannot accept the memory for other reason guest kernel will fallback to
> smaller size.
> 

Yes.