linux-kernel - Re: [PATCH v10 05/16] x86/virt/tdx: Add skeleton to enable TDX on demand

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Date:   Tue, 14 Mar 2023 10:38:33 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     "Huang, Kai" <kai.huang@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "david@...hat.com" <david@...hat.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>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Luck, Tony" <tony.luck@...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 v10 05/16] x86/virt/tdx: Add skeleton to enable TDX on
 demand

On 3/14/23 10:16, Isaku Yamahata wrote:
>>> TDX 1.5 spec introduced TDX_RND_NO_ENTROPY status code.  For TDX 1.0, let's
>>> postpone it to TDX 1.5 activity.
>> What the heck does this mean?
>>
>> I don't remember seeing any code here that checks for "TDX 1.0" or "TDX
>> 1.5".  That means that this code needs to work with _any_ TDX version.
>>
>> Are features being added to new versions that break code written for old
>> versions?
> No new feature, but new error code. TDX_RND_NO_ENTROPY, lack of entropy.
> For TDX 1.0, some APIs return TDX_SYS_BUSY. It can be contention(lock failure)
> or the lack of entropy.  The caller can't distinguish them.
> For TDX 1.5, they return TDX_RND_NO_ENTROPY instead of TDX_SYS_BUSY in the case
> of rdrand/rdseed failure.
> 
> Because both TDX_SYS_BUSY and TDX_RND_NO_ENTROPY are recoverable error
> (bit 63 error=1, bit 62 non_recoverable=0), the caller can check error bit and
> non_recoverable bit for retry.

Oh, that's actually really nice.  It separates out the "RDRAND is empty"
issue from the "the VMM should have had a lock here" issue.

For now, let's consider TDX_SYS_BUSY to basically indicate a non-fatal
kernel bug: the kernel called TDX in a way that it shouldn't have.
We'll treat it in the kernel as non-recoverable.  We'll return an error,
WARN_ON(), and keep on running.

A follow-on patch can add generic TDX_RND_NO_ENTROPY retry support to
the seamcall infrastructure.