Re: [PATCH v3 2/2] hw/arm/virt: Support for virtio-mem-pci

[Peter Maydell]

On Sat, 8 Jan 2022 at 07:22, Gavin Shan <gshan@redhat.com> wrote:
>
> Hi Peter,
>
> On 1/8/22 12:40 AM, Peter Maydell wrote:
> > On Fri, 3 Dec 2021 at 23:34, Gavin Shan <gshan@redhat.com> wrote:
> >> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> >> index b20595a496..21e4d572ab 100644
> >> --- a/hw/virtio/virtio-mem.c
> >> +++ b/hw/virtio/virtio-mem.c
> >> @@ -125,7 +125,7 @@ static uint64_t virtio_mem_default_block_size(RAMBlock 
> >> *rb)
> >>    * The memory block size corresponds mostly to the section size.
> >>    *
> >>    * This allows e.g., to add 20MB with a section size of 128MB on x86_64, 
> >> and
> >> - * a section size of 1GB on arm64 (as long as the start address is 
> >> properly
> >> + * a section size of 512MB on arm64 (as long as the start address is 
> >> properly
> >>    * aligned, similar to ordinary DIMMs).
> >>    *
> >>    * We can change this at any time and maybe even make it configurable if
> >> @@ -134,6 +134,8 @@ static uint64_t virtio_mem_default_block_size(RAMBlock 
> >> *rb)
> >>    */
> >>   #if defined(TARGET_X86_64) || defined(TARGET_I386)
> >>   #define VIRTIO_MEM_USABLE_EXTENT (2 * (128 * MiB))
> >> +#elif defined(TARGET_ARM)
> >> +#define VIRTIO_MEM_USABLE_EXTENT (2 * (512 * MiB))
> >>   #else
> >>   #error VIRTIO_MEM_USABLE_EXTENT not defined
> >>   #endif
> >
> > Could this comment explain where the 128MB and 512MB come from
> > and why the value is different for different architectures ?
> >
>
> Yes, the comment already explained it by "section size", which is the
> minimal hotpluggable unit. It's defined by the linux guest kernel as
> below. On ARM64, we pick the larger section size without considering
> the base page size. Besides, the virtio-mem is/will-be enabled on
> x86_64 and ARM64 guest kernel only.

Oh, so "section" is a Linux kernel concept? It wasn't clear to me
that that was a fixed value rather than something we were arbitrarily
defining in QEMU.

-- PMM