Re: [PULL 18/38] hw/arm/virt: Honor highmem setting when computing the memory map

[Akihiko Odaki <akihiko.odaki@gmail.com>]

On 2022/01/20 21:36, Peter Maydell wrote:
> From: Marc Zyngier <maz@kernel.org>
> 
> 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.
> 
> Reviewed-by: Andrew Jones <drjones@redhat.com>
> Reviewed-by: Eric Auger <eric.auger@redhat.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Message-id: 20220114140741.1358263-4-maz@kernel.org
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>   hw/arm/virt.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 62bdce1eb4b..3b839ba78ba 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -1670,7 +1670,7 @@ static uint64_t virt_cpu_mp_affinity(VirtMachineState *vms, int idx)
>   static void virt_set_memmap(VirtMachineState *vms)
>   {
>       MachineState *ms = MACHINE(vms);
> -    hwaddr base, device_memory_base, device_memory_size;
> +    hwaddr base, device_memory_base, device_memory_size, memtop;
>       int i;
>   
>       vms->memmap = extended_memmap;
> @@ -1697,7 +1697,11 @@ static void virt_set_memmap(VirtMachineState *vms)
>       device_memory_size = ms->maxram_size - ms->ram_size + ms->ram_slots * GiB;
>   
>       /* Base address of the high IO region */
> -    base = device_memory_base + ROUND_UP(device_memory_size, GiB);
> +    memtop = base = device_memory_base + ROUND_UP(device_memory_size, GiB);
> +    if (!vms->highmem && memtop > 4 * GiB) {
> +        error_report("highmem=off, but memory crosses the 4GiB limit\n");
> +        exit(EXIT_FAILURE);
> +    }
>       if (base < device_memory_base) {
>           error_report("maxmem/slots too huge");
>           exit(EXIT_FAILURE);
> @@ -1714,7 +1718,7 @@ 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 : memtop) - 1;
>       if (device_memory_size > 0) {
>           ms->device_memory = g_malloc0(sizeof(*ms->device_memory));
>           ms->device_memory->base = device_memory_base;

Hi,
This breaks in a case where highmem is disabled but can have more than 4 
GiB of RAM. M1 (Apple Silicon) actually can have 36-bit PA with HVF, 
which is not enough for highmem MMIO but is enough to contain 32 GiB of RAM.

Where the magic number of 4 GiB / 32-bit came from? I also don't quite 
understand what failures virt_kvm_type() had.

Regards,
Akihiko Odaki