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Re: [Qemu-devel] [PATCH] intel_iommu: do address space switching when re

From: Alex Williamson
Subject: Re: [Qemu-devel] [PATCH] intel_iommu: do address space switching when reset
Date: Wed, 5 Sep 2018 08:55:50 -0600

On Wed,  5 Sep 2018 19:31:58 +0800
Peter Xu <address@hidden> wrote:

> We will drop all the mappings when system reset, however we'll still
> keep the existing memory layouts.  That'll be problematic since if IOMMU
> is enabled in the guest and then reboot the guest, SeaBIOS will try to
> drive a device that with no page mapped there.  What we need to do is to
> rebuild the GPA->HPA mapping when system resets, hence ease SeaBIOS.
> Without this patch, a guest that boots on an assigned NVMe device might
> fail to find the boot device after a system reboot/reset and we'll be
> able to observe SeaBIOS errors if turned on:
>   WARNING - Timeout at nvme_wait:144!
> With the patch applied, the guest will be able to find the NVMe drive
> and bootstrap there even after multiple reboots or system resets.
> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1625173
> CC: QEMU Stable <address@hidden>
> Tested-by: Cong Li <address@hidden>
> Signed-off-by: Peter Xu <address@hidden>
> ---
>  hw/i386/intel_iommu.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> index 3dfada19a6..d3eb068d43 100644
> --- a/hw/i386/intel_iommu.c
> +++ b/hw/i386/intel_iommu.c
> @@ -3231,6 +3231,14 @@ static void vtd_reset(DeviceState *dev)
>       * When device reset, throw away all mappings and external caches
>       */
>      vtd_address_space_unmap_all(s);
> +
> +    /*
> +     * Switch address spaces if needed (e.g., when reboot from a
> +     * kernel that has IOMMU enabled, we should switch address spaces
> +     * to rebuild the GPA->HPA mappings otherwise SeaBIOS might
> +     * encounter DMA errors when running with e.g. a NVMe card).
> +     */
> +    vtd_switch_address_space_all(s);
>  }
>  static AddressSpace *vtd_host_dma_iommu(PCIBus *bus, void *opaque, int devfn)

I'm curious why these aren't part of vtd_init().  vtd_init is where
GCMD is set back to it's power-on state, which disables translation, so
logically we should reset the address space at that point.  Similarly,
the root entry is reset, so it would make sense to throw away all the
mappings there too.  Thanks,


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