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Re: [Xen-devel] [v7][PATCH 07/16] hvmloader/e820: construct guest e820 table

On 2015/7/14 17:32, Jan Beulich wrote:
On 14.07.15 at 07:22, <tiejun.chen@xxxxxxxxx> wrote:
+    for ( i = 0; i < memory_map.nr_map; i++ )
+    {
+        uint64_t end = e820[i].addr + e820[i].size;

Either loop index/boundary or used array are wrong here: In the
earlier loop you copied memory_map[0...nr_map-1] to
e820[n...n+nr_map-1], but here you're looping looking at

You're right. I should lookup all e820[] like this,

for ( i = 0; i < nr; i++ )

Hmm, I would have thought you only care about either part of
the just glued together map.

+        if ( e820[i].type == E820_RAM &&
+             low_mem_end > e820[i].addr && low_mem_end < end )

Assuming you mean to look at the RDM e820[] entries here, this
is not a correct check: You don't care about partly or fully
contained, all you care about is whether low_mem_end extends
beyond the start of the region.

Here I'm looking at the e820 entry indicating low memory. Because

low_mem_end = hvm_info->low_mem_pgend << PAGE_SHIFT;

and when we allocate MMIO in pci.c, its possible to populate RAM so
hvm_info->low_mem_pgend would be changed over there. So we need to
compensate this loss with high memory. Here memory_map[] also records
the original low/high memory, so if low_mem_end is less-than the
original we need this compensation.

And I'm not disputing your intentions - I'm merely pointing out that
afaics the code above doesn't match these intentions. In particular
(as said) I don't see why you need to check low_mem_end < end.

Before we probably relocate RAM,

low_mem_end = hvm_info->low_mem_pgend << PAGE_SHIFT

and the e820 entry specific to low memory,

[e820[X].addr, end]

Here, end = e820[X].addr + e820[X].size;

Now low_mem_end = end.

After that, low_mem_end < end. so if

(low_mem_end > e820[X].addr && low_mem_end < end) is true, this means that associated RAM entry is hitting, right? Then we need to revise this entry as [e820[X].addr, low_mem_end], and compensate [end - low_mem_end] to high memory. Anything I'm still wrong here?


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