linux-kernel - Re: [PATCH v3 13/21] x86/virt/tdx: Allocate and set up PAMTs for TDMRs

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Message-ID: <8d5715b5-d561-f482-af11-03a9a46e651a@intel.com>
Date:   Mon, 2 May 2022 07:17:48 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, pbonzini@...hat.com, len.brown@...el.com,
        tony.luck@...el.com, rafael.j.wysocki@...el.com,
        reinette.chatre@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v3 13/21] x86/virt/tdx: Allocate and set up PAMTs for
 TDMRs

On 5/1/22 22:59, Kai Huang wrote:
> On Fri, 2022-04-29 at 07:20 -0700, Dave Hansen wrote:
> How about adding below in the changelog:
> 
> "
> However using alloc_contig_pages() to allocate large physically contiguous
> memory at runtime may fail.  The larger the allocation, the more likely it is to
> fail.  Due to the fragmentation, the kernel may need to move pages out of the
> to-be-allocated contiguous memory range but it may fail to move even the last
> stubborn page.  A good way (although not foolproof) is to launch a TD VM early
> in boot to get PAMTs allocated before memory gets fragmented or consumed.
> "

Better, although it's getting a bit off topic for this changelog.

Just be short and sweet:

1. the allocation can fail
2. Launch a VM early to (badly) mitigate this
3. the only way to fix it is to add a boot option


>>>>> +	/*
>>>>> +	 * One TDMR must cover at least one (or partial) RAM entry,
>>>>> +	 * otherwise it is kernel bug.  WARN_ON() in this case.
>>>>> +	 */
>>>>> +	if (WARN_ON_ONCE((start >= end) || start >= TDMR_END(tdmr)))
>>>>> +		return 0;
>>
>> This really means "no RAM found for this TDMR", right?  Can we say that,
>> please.
> 
> OK will add it.  How about:
> 
> 	/*
> 	 * No RAM found for this TDMR.  WARN() in this case, as it
> 	 * cannot happen otherwise it is a kernel bug.
> 	 */

The only useful information in that comment is the first sentence.  The
jibberish about WARN() is patently obvious from the next two lines of code.

*WHY* can't this happen?  How might it have actually happened?