Re: [PATCH v2 3/5] hw/arm/virt: Honor highmem setting when computing the memory map

From: Eric Auger <eric.auger@redhat.com>
To: Marc Zyngier <maz@kernel.org>, qemu-devel@nongnu.org
Cc: Peter Maydell <peter.maydell@linaro.org>,
	Andrew Jones <drjones@redhat.com>,
	kernel-team@android.com, kvmarm@lists.cs.columbia.edu,
	kvm@vger.kernel.org
Subject: Re: [PATCH v2 3/5] hw/arm/virt: Honor highmem setting when computing the memory map
Date: Mon, 4 Oct 2021 14:23:41 +0200	[thread overview]
Message-ID: <b36a602e-a8f4-c8ac-bd4b-95fd6d426736@redhat.com> (raw)
In-Reply-To: <20211003164605.3116450-4-maz@kernel.org>

Hi Marc,

On 10/3/21 6:46 PM, Marc Zyngier wrote:
> Even when the VM is configured with highmem=off, the highest_gpa
> field includes devices that are above the 4GiB limit.
> Similarily, nothing seem to check that the memory is within
> the limit set by the highmem=off option.
>
> This leads to failures in virt_kvm_type() on systems that have
> a crippled IPA range, as the reported IPA space is larger than
> what it should be.
>
> Instead, honor the user-specified limit to only use the devices
> at the lowest end of the spectrum, and fail if we have memory
> crossing the 4GiB limit.
>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  hw/arm/virt.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index bcf58f677d..9d2abdbd5f 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -1628,6 +1628,11 @@ static void virt_set_memmap(VirtMachineState *vms)
>          exit(EXIT_FAILURE);
>      }
>  
> +    if (!vms->highmem &&
> +        vms->memmap[VIRT_MEM].base + ms->maxram_size > 4 * GiB) {
> +        error_report("highmem=off, but memory crosses the 4GiB limit\n");
> +        exit(EXIT_FAILURE);
> +    }
>      /*
>       * We compute the base of the high IO region depending on the
>       * amount of initial and device memory. The device memory start/size
> @@ -1657,7 +1662,9 @@ static void virt_set_memmap(VirtMachineState *vms)
>          vms->memmap[i].size = size;
>          base += size;
>      }
> -    vms->highest_gpa = base - 1;
> +    vms->highest_gpa = (vms->highmem ?
> +                        base :
> +                        vms->memmap[VIRT_MEM].base + ms->maxram_size) - 1;
I think I would have preferred to have

if (vms->highmem) {
   for (i = VIRT_LOWMEMMAP_LAST; i < ARRAY_SIZE(extended_memmap); i++) {
        hwaddr size = extended_memmap[i].size;

        base = ROUND_UP(base, size);
        vms->memmap[i].base = base;
        vms->memmap[i].size = size;
        base += size;
    }
}
as it is useless to execute that code and create new memmap entries in
case of !highmem.

But nevertheless, this looks correct

Eric
>      if (device_memory_size > 0) {
>          ms->device_memory = g_malloc0(sizeof(*ms->device_memory));
>          ms->device_memory->base = device_memory_base;