Re: [PATCH v2 kvmtool 29/30] vfio: Trap MMIO access to BAR addresses which aren't page aligned

From: Andre Przywara <andre.przywara@arm.com>
To: Alexandru Elisei <alexandru.elisei@arm.com>
Cc: kvm@vger.kernel.org, will@kernel.org,
	julien.thierry.kdev@gmail.com, sami.mujawar@arm.com,
	lorenzo.pieralisi@arm.com, maz@kernel.org
Subject: Re: [PATCH v2 kvmtool 29/30] vfio: Trap MMIO access to BAR addresses which aren't page aligned
Date: Fri, 7 Feb 2020 16:51:23 +0000	[thread overview]
Message-ID: <20200207165123.4fdd7111@donnerap.cambridge.arm.com> (raw)
In-Reply-To: <20200123134805.1993-30-alexandru.elisei@arm.com>

On Thu, 23 Jan 2020 13:48:04 +0000
Alexandru Elisei <alexandru.elisei@arm.com> wrote:

Hi,

> KVM_SET_USER_MEMORY_REGION will fail if the guest physical address is
> not aligned to the page size. However, it is legal for a guest to
> program an address which isn't aligned to the page size. Trap and
> emulate MMIO accesses to the region when that happens.
> 
> Without this patch, when assigning a Seagate Barracude hard drive to a
> VM I was seeing these errors:
> 
> [    0.286029] pci 0000:00:00.0: BAR 0: assigned [mem 0x41004600-0x4100467f]
>   Error: 0000:01:00.0: failed to register region with KVM
>   Error: [1095:3132] Error activating emulation for BAR 0
> [..]
> [   10.561794] irq 13: nobody cared (try booting with the "irqpoll" option)
> [   10.563122] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.4.0-seattle-00009-g909b20467ed1 #133
> [   10.563124] Hardware name: linux,dummy-virt (DT)
> [   10.563126] Call trace:
> [   10.563134]  dump_backtrace+0x0/0x140
> [   10.563137]  show_stack+0x14/0x20
> [   10.563141]  dump_stack+0xbc/0x100
> [   10.563146]  __report_bad_irq+0x48/0xd4
> [   10.563148]  note_interrupt+0x288/0x378
> [   10.563151]  handle_irq_event_percpu+0x80/0x88
> [   10.563153]  handle_irq_event+0x44/0xc8
> [   10.563155]  handle_fasteoi_irq+0xb4/0x160
> [   10.563157]  generic_handle_irq+0x24/0x38
> [   10.563159]  __handle_domain_irq+0x60/0xb8
> [   10.563162]  gic_handle_irq+0x50/0xa0
> [   10.563164]  el1_irq+0xb8/0x180
> [   10.563166]  arch_cpu_idle+0x10/0x18
> [   10.563170]  do_idle+0x204/0x290
> [   10.563172]  cpu_startup_entry+0x20/0x40
> [   10.563175]  rest_init+0xd4/0xe0
> [   10.563180]  arch_call_rest_init+0xc/0x14
> [   10.563182]  start_kernel+0x420/0x44c
> [   10.563183] handlers:
> [   10.563650] [<000000001e474803>] sil24_interrupt
> [   10.564559] Disabling IRQ #13
> [..]
> [   11.832916] ata1: spurious interrupt (slot_stat 0x0 active_tag -84148995 sactive 0x0)
> [   12.045444] ata_ratelimit: 1 callbacks suppressed
> 
> With this patch, I don't see the errors and the device works as
> expected.

Pretty neat and easy fix for that nasty problem!

> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>

Reviewed-by: Andre Przywara <andre.przywara@arm.com>

Cheers,
Andre.

> ---
>  vfio/core.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/vfio/core.c b/vfio/core.c
> index 6b9b58ea8d2f..b23e77c54771 100644
> --- a/vfio/core.c
> +++ b/vfio/core.c
> @@ -226,6 +226,15 @@ int vfio_map_region(struct kvm *kvm, struct vfio_device *vdev,
>  	if (!(region->info.flags & VFIO_REGION_INFO_FLAG_MMAP))
>  		return vfio_setup_trap_region(kvm, vdev, region);
>  
> +	/*
> +	 * KVM_SET_USER_MEMORY_REGION will fail because the guest physical
> +	 * address isn't page aligned, let's emulate the region ourselves.
> +	 */
> +	if (region->guest_phys_addr & (PAGE_SIZE - 1))
> +		return kvm__register_mmio(kvm, region->guest_phys_addr,
> +					  region->info.size, false,
> +					  vfio_mmio_access, region);
> +
>  	if (region->info.flags & VFIO_REGION_INFO_FLAG_READ)
>  		prot |= PROT_READ;
>  	if (region->info.flags & VFIO_REGION_INFO_FLAG_WRITE)