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X-MS-Exchange-CrossTenant-Id: 335836de-42ef-43a2-b145-348c2ee9ca5b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: nb170M4+s1uheOmp+e5TioG2rQjxhz3jbSKB57FKIJh2ujuNPrazcw8OfmUPpkncEXT3zVqa1pdHiBrCcIicHJpQem5lb/SEKdf2g1Q4MOg= X-MS-Exchange-Transport-CrossTenantHeadersStamped: MN2PR03MB5295 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19/05/2023 1:41 pm, Christoph Hellwig wrote: > On Fri, May 19, 2023 at 12:10:26PM +0200, Marek Marczykowski-Górecki wrote: >> While I would say PCI passthrough is not very common for PV guests, can >> the decision about xen-swiotlb be delayed until you can enumerate >> xenstore to check if there are any PCI devices connected (and not >> allocate xen-swiotlb by default if there are none)? This would >> still not cover the hotplug case (in which case, you'd need to force it >> with a cmdline), but at least you wouldn't loose much memory just >> because one of your VMs may use PCI passthrough (so, you have it enabled >> in your kernel). > How early can we query xenstore? We'd need to do this before setting > up DMA for any device. Not that early.  One supported configuration has xenstore not starting for an indefinite period of time after boot. > The alternative would be to finally merge swiotlb-xen into swiotlb, in > which case we might be able to do this later. Let me see what I can > do there. If that is an option, it would be great to reduce the special-cashing. ~Andrew