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Re: [PATCH 11/15] x86/PVH: try to map initrd directly into Dom0
- To: Roger Pau Monné <roger@xxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 5 Nov 2026 09:06:15 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 05 Nov 2026 08:06:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 15.10.2026 18:32, Roger Pau Monné wrote:
> On Mon, Oct 12, 2026 at 10:25:11AM +0200, Jan Beulich wrote:
>> Why does find_memory() go upwards, while find_kernel_memory() goes
>> downwards? And why do we use the lowest part of the region found by
>> find_memory() rather than the highest part?
>
> find_memory() should probably start downwards from 4G.
In the course of replacing that find_memory() by pvh_steal_ram() (while
converting to rangeset-based tracking) I learned the hard way that this
isn't possible, even if it ought to be. Linux won't cope with parts of
start-info (at least the module array) living outside of the first Gb
(in 32-bit Linux it may go farther, depending on CONFIG_PAGE_OFFSET).
See pvh_level3_ident_pgt[] having only its first direct-map entry
populated, and pvh_start_xen() also only updating that one entry. Which
causes xen_prepare_pvh() -> init_pvh_bootparams() to fall over anything
living beyond the 1Gb boundary. (.memmap_paddr would apparently have
the same issue, just that for Dom0 we don't set that.)
Jan
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