Re: [Qemu-devel] [PATCH v10 10/10] hw/arm/virt: Bump the 255GB initial RAM limit

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 1 Mar 2019 11:08:17 +0100
Auger Eric <eric.auger@redhat.com> wrote:

> Hi Igor,
> 
> On 2/28/19 5:29 PM, Igor Mammedov wrote:
> > On Thu, 28 Feb 2019 16:03:24 +0100
> > Eric Auger <eric.auger@redhat.com> wrote:
> >   
> >> Now we have the extended memory map (high IO regions beyond the
> >> scalable RAM) and dynamic IPA range support at KVM/ARM level
> >> we can bump the legacy 255GB initial RAM limit. The actual maximum
> >> RAM size now depends on the physical CPU and host kernel, in
> >> accelerated mode. In TCG mode, it depends on the VCPU
> >> AA64MMFR0.PARANGE.
> >>
> >> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> >>
> >> ---
> >> v7 -> v8:
> >> - TCG PAMAX check moved in a separate patch
> >>
> >> v6 -> v7
> >> - handle TCG case
> >> - set_memmap modifications moved to previous patches
> >> ---
> >>  hw/arm/virt.c | 21 +--------------------
> >>  1 file changed, 1 insertion(+), 20 deletions(-)
> >>
> >> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> >> index a3da75a5ae..a45f0fcf79 100644
> >> --- a/hw/arm/virt.c
> >> +++ b/hw/arm/virt.c
> >> @@ -95,21 +95,8 @@
> >>  
> >>  #define PLATFORM_BUS_NUM_IRQS 64
> >>  
> >> -/* RAM limit in GB. Since VIRT_MEM starts at the 1GB mark, this means
> >> - * RAM can go up to the 256GB mark, leaving 256GB of the physical
> >> - * address space unallocated and free for future use between 256G and 512G.
> >> - * If we need to provide more RAM to VMs in the future then we need to:
> >> - *  * allocate a second bank of RAM starting at 2TB and working up
> >> - *  * fix the DT and ACPI table generation code in QEMU to correctly
> >> - *    report two split lumps of RAM to the guest
> >> - *  * fix KVM in the host kernel to allow guests with >40 bit address spaces
> >> - * (We don't want to fill all the way up to 512GB with RAM because
> >> - * we might want it for non-RAM purposes later. Conversely it seems
> >> - * reasonable to assume that anybody configuring a VM with a quarter
> >> - * of a terabyte of RAM will be doing it on a host with more than a
> >> - * terabyte of physical address space.)
> >> - */
> >>  #define RAMBASE GiB
> >> +/* Legacy RAM limit in GB (< version 4.0) */
> >>  #define LEGACY_RAMLIMIT_GB 255
> >>  #define LEGACY_RAMLIMIT_BYTES (LEGACY_RAMLIMIT_GB * GiB)  
> > do we need to keep these couple around?
> > 
> > it's used only in
> >  [VIRT_MEM] = { RAMBASE, LEGACY_RAMLIMIT_BYTES },
> > and doesn't have any effect whatsoever.
> > I'd set initial  VIRT_MEM.size to 0 and drop LEGACY_RAMLIMIT_*
> > maybe add comment above entry that size is defined by ram_size  
> 
> in virt_set_memmap I was checking if (high_io_base < 256 GiB) then
> high_io_base = 256GiB. Maybe this 256GiB value comes out of the blue and
> I should also replace it with vms->memmap[VIRT_MEM].base +
> LEGACY_RAMLIMIT_BYTES.
I'd go for it + comment on top of it.

> We maintain some kind of compatibility with the old memmap so I prefer
> to keep this info somewhere.
> 
> I added a comment though:
> /* Actual RAM size depends on initial RAM and device memory options */
> 
> Thanks
> 
> Eric
> 
> 
> >   
> >>  
> >> @@ -1515,12 +1502,6 @@ static void machvirt_init(MachineState *machine)
> >>  
> >>      vms->smp_cpus = smp_cpus;
> >>  
> >> -    if (machine->ram_size > vms->memmap[VIRT_MEM].size) {
> >> -        error_report("mach-virt: cannot model more than %dGB RAM",
> >> -                     LEGACY_RAMLIMIT_GB);
> >> -        exit(1);
> >> -    }
> >> -
> >>      if (vms->virt && kvm_enabled()) {
> >>          error_report("mach-virt: KVM does not support providing "
> >>                       "Virtualization extensions to the guest CPU");  
> >