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SubjectRe: [PATCH v5 12/22] x86/virt/tdx: Convert all memory regions in memblock to TDX memory
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On Thu, 2022-07-07 at 16:36 +0200, Juergen Gross wrote:
> On 07.07.22 16:26, Dave Hansen wrote:
> > On 6/26/22 23:16, Kai Huang wrote:
> > > On Fri, 2022-06-24 at 12:40 -0700, Dave Hansen wrote:
> > > > > +/*
> > > > > + * Walks over all memblock memory regions that are intended to be
> > > > > + * converted to TDX memory. Essentially, it is all memblock memory
> > > > > + * regions excluding the low memory below 1MB.
> > > > > + *
> > > > > + * This is because on some TDX platforms the low memory below 1MB is
> > > > > + * not included in CMRs. Excluding the low 1MB can still guarantee
> > > > > + * that the pages managed by the page allocator are always TDX memory,
> > > > > + * as the low 1MB is reserved during kernel boot and won't end up to
> > > > > + * the ZONE_DMA (see reserve_real_mode()).
> > > > > + */
> > > > > +#define memblock_for_each_tdx_mem_pfn_range(i, p_start, p_end, p_nid) \
> > > > > + for_each_mem_pfn_range(i, MAX_NUMNODES, p_start, p_end, p_nid) \
> > > > > + if (!pfn_range_skip_lowmem(p_start, p_end))
> > > >
> > > > Let's summarize where we are at this point:
> > > >
> > > > 1. All RAM is described in memblocks
> > > > 2. Some memblocks are reserved and some are free
> > > > 3. The lower 1MB is marked reserved
> > > > 4. for_each_mem_pfn_range() walks all reserved and free memblocks, so we
> > > > have to exclude the lower 1MB as a special case.
> > > >
> > > > That seems superficially rather ridiculous. Shouldn't we just pick a
> > > > memblock iterator that skips the 1MB? Surely there is such a thing.
> > >
> > > Perhaps you are suggesting we should always loop the _free_ ranges so we don't
> > > need to care about the first 1MB which is reserved?
> > >
> > > The problem is some reserved memory regions are actually later freed to the page
> > > allocator, for example, initrd. So to cover all those 'late-freed-reserved-
> > > regions', I used for_each_mem_pfn_range(), instead of for_each_free_mem_range().
> >
> > Why not just entirely remove the lower 1MB from the memblock structure
> > on TDX systems? Do something equivalent to adding this on the kernel
> > command line:
> >
> > memmap=1M$0x0
> >
> > > Btw, I do have a checkpatch warning around this code:
> > >
> > > ERROR: Macros with complex values should be enclosed in parentheses
> > > #109: FILE: arch/x86/virt/vmx/tdx/tdx.c:377:
> > > +#define memblock_for_each_tdx_mem_pfn_range(i, p_start, p_end, p_nid) \
> > > + for_each_mem_pfn_range(i, MAX_NUMNODES, p_start, p_end, p_nid) \
> > > + if (!pfn_range_skip_lowmem(p_start, p_end))
> > >
> > > But it looks like a false positive to me.
> >
> > I think it doesn't like the if().
>
> I think it is right.
>
> Consider:
>
> if (a)
> memblock_for_each_tdx_mem_pfn_range(...)
> func();
> else
> other_func();
>
>
>
Interesting case. Thanks.

Yes we will require explicit { } around memblock_for_each_tdx_mem_pfn_range() in
this case.

--
Thanks,
-Kai


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