Re: [PATCH] hw/riscv: split RAM into low and high memory

[Andrew Jones]

On Mon, Jul 31, 2023 at 09:53:17AM +0800, Fei Wu wrote:
> riscv virt platform's memory started at 0x80000000 and
> straddled the 4GiB boundary. Curiously enough, this choice
> of a memory layout will prevent from launching a VM with
> a bit more than 2000MiB and PCIe pass-thru on an x86 host, due
> to identity mapping requirements for the MSI doorbell on x86,
> and these (APIC/IOAPIC) live right below 4GiB.
> 
> So just split the RAM range into two portions:
> - 1 GiB range from 0x80000000 to 0xc0000000.
> - The remainder at 0x100000000
> 
> ...leaving a hole between the ranges.

Can you elaborate on the use case? Maybe provide details of the host
system and the QEMU command line? I'm wondering why we didn't have
any problems with the arm virt machine type. Has nobody tried this
use case with that? Is the use case something valid for riscv, but
not arm?

Thanks,
drew