Re: [PATCH v6 6/7] hw/arm/virt: Add 'compact-highmem' property

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Oct 24 2022, Gavin Shan <gshan@redhat.com> wrote:

> After the improvement to high memory region address assignment is
> applied, the memory layout can be changed, introducing possible
> migration breakage. For example, VIRT_HIGH_PCIE_MMIO memory region
> is disabled or enabled when the optimization is applied or not, with
> the following configuration. The configuration is only achievable by
> modifying the source code until more properties are added to allow
> users selectively disable those high memory regions.
>
>   pa_bits              = 40;
>   vms->highmem_redists = false;
>   vms->highmem_ecam    = false;
>   vms->highmem_mmio    = true;
>
>   # qemu-system-aarch64 -accel kvm -cpu host    \
>     -machine virt-7.2,compact-highmem={on, off} \
>     -m 4G,maxmem=511G -monitor stdio
>
>   Region             compact-highmem=off         compact-highmem=on
>   ----------------------------------------------------------------
>   MEM                [1GB         512GB]        [1GB         512GB]
>   HIGH_GIC_REDISTS2  [512GB       512GB+64MB]   [disabled]
>   HIGH_PCIE_ECAM     [512GB+256MB 512GB+512MB]  [disabled]
>   HIGH_PCIE_MMIO     [disabled]                 [512GB       1TB]
>
> In order to keep backwords compatibility, we need to disable the
> optimization on machine, which is virt-7.1 or ealier than it. It
> means the optimization is enabled by default from virt-7.2. Besides,
> 'compact-highmem' property is added so that the optimization can be
> explicitly enabled or disabled on all machine types by users.
>
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> Reviewed-by: Cornelia Huck <cohuck@redhat.com>
> Tested-by: Zhenyu Zhang <zhenyzha@redhat.com>
> ---
>  docs/system/arm/virt.rst |  4 ++++
>  hw/arm/virt.c            | 32 ++++++++++++++++++++++++++++++++
>  include/hw/arm/virt.h    |  1 +
>  3 files changed, 37 insertions(+)
>

(...)

> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 4896f600b4..11b5685432 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -174,6 +174,12 @@ static const MemMapEntry base_memmap[] = {
>   * Note the extended_memmap is sized so that it eventually also includes the
>   * base_memmap entries (VIRT_HIGH_GIC_REDIST2 index is greater than the last
>   * index of base_memmap).
> + *
> + * The memory map for these Highmem IO Regions can be in legacy or compact
> + * layout, depending on 'compact-highmem' property. With legacy layout, the
> + * PA space for one specific region is always reserved, even the region has

s/even/even if/

> + * been disabled or doesn't fit into the PA space. However, the PA space for
> + * the region won't be reserved in these circumstances with compact layout.
>   */
>  static MemMapEntry extended_memmap[] = {
>      /* Additional 64 MB redist region (can contain up to 512 redistributors) */