linux-kernel - Re: [PATCH 2/4] x86: always initialize xen-swiotlb when xen-pcifront is enabling

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Date:   Mon, 12 Jun 2023 10:08:07 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        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>, Ben Skeggs <bskeggs@...hat.com>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>, xen-devel@...ts.xenproject.org,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
        nouveau@...ts.freedesktop.org
Subject: Re: [PATCH 2/4] x86: always initialize xen-swiotlb when xen-pcifront
 is enabling

On 09.06.23 17:38, Juergen Gross wrote:
> On 07.06.23 15:12, Christoph Hellwig wrote:
>> On Mon, May 22, 2023 at 10:37:09AM +0200, Juergen Gross wrote:
>>> In normal cases PCI passthrough in PV guests requires to start the guest
>>> with e820_host=1. So it should be rather easy to limit allocating the
>>> 64MB in PV guests to the cases where the memory map has non-RAM regions
>>> especially in the first 1MB of the memory.
>>>
>>> This will cover even hotplug cases. The only case not covered would be a
>>> guest started with e820_host=1 even if no PCI passthrough was planned.
>>> But this should be rather rare (at least I hope so).
>>
>> So is this an ACK for the patch and can we go ahead with it?
> 
> As long as above mentioned check of the E820 map is done, yes.
> 
> If you want I can send a diff to be folded into your patch on Monday.

Attached is a patch for adding a new flag xen_pv_pci_possible. You can
either add the patch to your series or merge it into your patch 2.

You need to modify your patch like this:

@@ -111,7 +90,10 @@ static inline void __init pci_xen_swiotlb_init(void)
  void __init pci_iommu_alloc(void)
  {
  	if (xen_pv_domain()) {
-		if (xen_initial_domain() || x86_swiotlb_enable)
+		if (xen_initial_domain() ||
+		    (IS_ENABLED(CONFIG_XEN_PCIDEV_FRONTEND) &&
+		     xen_pv_pci_possible) ||
+		    x86_swiotlb_enable)
  			pci_xen_swiotlb_init();
  		return;
  	}


Juergen

View attachment "0001-xen-pci-add-flag-for-PCI-passthrough-being-possible.patch" of type "text/x-patch" (1903 bytes)

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