linux-kernel - Re: [PATCH RFC] x86/xen: attempt to inflate the memory balloon on PVH

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Message-ID: <48f416e8-bc9a-40ba-b2d6-80c047ba26e4@suse.com>
Date: Wed, 13 Mar 2024 16:04:25 +0100
From: Juergen Gross <jgross@...e.com>
To: Roger Pau Monne <roger.pau@...rix.com>, xen-devel@...ts.xenproject.org
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 linux-kernel@...r.kernel.org, Jan Beulich <jbeulich@...e.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH RFC] x86/xen: attempt to inflate the memory balloon on PVH

On 20.02.24 18:43, Roger Pau Monne wrote:
> When running as PVH or HVM Linux will use holes in the memory map as scratch
> space to map grants, foreign domain pages and possibly miscellaneous other
> stuff.  However the usage of such memory map holes for Xen purposes can be
> problematic.  The request of holesby Xen happen quite early in the kernel boot
> process (grant table setup already uses scratch map space), and it's possible
> that by then not all devices have reclaimed their MMIO space.  It's not
> unlikely for chunks of Xen scratch map space to end up using PCI bridge MMIO
> window memory, which (as expected) causes quite a lot of issues in the system.
> 
> At least for PVH dom0 we have the possibility of using regions marked as
> UNUSABLE in the e820 memory map.  Either if the region is UNUSABLE in the
> native memory map, or it has been converted into UNUSABLE in order to hide RAM
> regions from dom0, the second stage translation page-tables can populate those
> areas without issues.
> 
> PV already has this kind of logic, where the balloon driver is inflated at
> boot.  Re-use the current logic in order to also inflate it when running as
> PVH.  onvert UNUSABLE regions up to the ratio specified in EXTRA_MEM_RATIO to
> RAM, while reserving them using xen_add_extra_mem() (which is also moved so
> it's no longer tied to CONFIG_PV).
> 
> Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>

Reviewed-by: Juergen Gross <jgross@...e.com>

Juergen

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