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SubjectRe: [PATCH v3 13/21] x86/virt/tdx: Allocate and set up PAMTs for TDMRs
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On Mon, 2022-05-02 at 07:17 -0700, Dave Hansen wrote:
> 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
>
OK. Thanks.

>
> > > > > > + /*
> > > > > > + * 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?

When TDMRs are created, we already have made sure one TDMR must cover at least
one or partial RAM entry.

--
Thanks,
-Kai


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