linux-kernel - Re: [RFC PATCH 1/5] x86/xen: disable swiotlb for xen pvh

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Message-ID: <ZBNDyB7Jp0rgjZNy@Air-de-Roger>
Date:   Thu, 16 Mar 2023 17:28:56 +0100
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Huang Rui <ray.huang@....com>
Cc:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, amd-gfx@...ts.freedesktop.org,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        Stewart Hildebrand <Stewart.Hildebrand@....com>,
        Xenia Ragiadakou <burzalodowa@...il.com>,
        Honglei Huang <honglei1.huang@....com>,
        Julia Zhang <julia.zhang@....com>,
        Chen Jiqian <Jiqian.Chen@....com>
Subject: Re: [RFC PATCH 1/5] x86/xen: disable swiotlb for xen pvh

On Sun, Mar 12, 2023 at 08:01:53PM +0800, Huang Rui wrote:
> Xen PVH is the paravirtualized mode and takes advantage of hardware
> virtualization support when possible. It will using the hardware IOMMU
> support instead of xen-swiotlb, so disable swiotlb if current domain is
> Xen PVH.
> 
> Signed-off-by: Huang Rui <ray.huang@....com>
> ---
>  arch/x86/kernel/pci-dma.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
> index 30bbe4abb5d6..f5c73dd18f2a 100644
> --- a/arch/x86/kernel/pci-dma.c
> +++ b/arch/x86/kernel/pci-dma.c
> @@ -74,6 +74,12 @@ static inline void __init pci_swiotlb_detect(void)
>  #ifdef CONFIG_SWIOTLB_XEN
>  static void __init pci_xen_swiotlb_init(void)
>  {
> +	/* Xen PVH domain won't use swiotlb */
> +	if (xen_pvh_domain()) {
> +		x86_swiotlb_enable = false;
> +		return;
> +	}

I'm very confused by this: pci_xen_swiotlb_init() is only called for
PV domains, see the only caller in pci_iommu_alloc().  So this is just
dead code.

> +
>  	if (!xen_initial_domain() && !x86_swiotlb_enable)
>  		return;
>  	x86_swiotlb_enable = true;
> @@ -86,7 +92,7 @@ static void __init pci_xen_swiotlb_init(void)
>  
>  int pci_xen_swiotlb_init_late(void)
>  {
> -	if (dma_ops == &xen_swiotlb_dma_ops)
> +	if (xen_pvh_domain() || dma_ops == &xen_swiotlb_dma_ops)

Same here, this function is only called by
pcifront_connect_and_init_dma() and pcifront should never attach on a
PVH domain, hence it's also dead code.

Thanks, Roger.