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

["Roger Pau Monné" <roger.pau@citrix.com>]

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@amd.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.