linux-kernel - Re: [PATCH v7 17/20] x86/virt/tdx: Configure global KeyID on all packages

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Date:   Wed, 30 Nov 2022 07:13:18 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Huang, Kai" <kai.huang@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>
Cc:     "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>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "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>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v7 17/20] x86/virt/tdx: Configure global KeyID on all
 packages

On 11/30/22 00:34, Huang, Kai wrote:
> On Wed, 2022-11-30 at 11:35 +0800, Binbin Wu wrote:
>> On 11/21/2022 8:26 AM, Kai Huang wrote:
>>> After the array of TDMRs and the global KeyID are configured to the TDX
>>> module, use TDH.SYS.KEY.CONFIG to configure the key of the global KeyID
>>> on all packages.
>>>
>>> TDH.SYS.KEY.CONFIG must be done on one (any) cpu for each package.  And
>>> it cannot run concurrently on different CPUs.  Implement a helper to
>>> run SEAMCALL on one cpu for each package one by one, and use it to
>>> configure the global KeyID on all packages.
>>>
>>> Intel hardware doesn't guarantee cache coherency across different
>>> KeyIDs.  The kernel needs to flush PAMT's dirty cachelines (associated
>>> with KeyID 0) before the TDX module uses the global KeyID to access the
>>> PAMT.  Following the TDX module specification, flush cache before
>>> configuring the global KeyID on all packages.
>>>
>>> Given the PAMT size can be large (~1/256th of system RAM), just use
>>> WBINVD on all CPUs to flush.
>>>
>>> Note if any TDH.SYS.KEY.CONFIG fails, the TDX module may already have
>>> used the global KeyID to write any PAMT.  Therefore, need to use WBINVD
>>> to flush cache before freeing the PAMTs back to the kernel.  Note using
>>> MOVDIR64B (which changes the page's associated KeyID from the old TDX
>>> private KeyID back to KeyID 0, which is used by the kernel)
>>
>> It seems not accurate to say MOVDIR64B changes the page's associated KeyID.
>> It just uses the current KeyID for memory operations.
> 
> The "write" to the memory changes the page's associated KeyID to the KeyID that
> does the "write".  A more accurate expression perhaps should be MOVDIR64B +
> MFENSE, but I think it doesn't matter in changelog.

Just delete it from the changelog.  It's a distraction.  I'm not even
sure it's *necessary* to do any memory content conversion after the TDX
module has written gunk.

There won't be any integrity issues because integrity errors don't do
anything for KeyID-0 (no #MC).

I _think_ the reads of the page using KeyID-0 will see abort page
semantics.  That's *FINE*.