Re: [PATCH RFCv2 3/4] i386/pc: warn if phys-bits is too low

[David Edmondson <david.edmondson@oracle.com>]

On Monday, 2022-02-07 at 20:24:21 GMT, Joao Martins wrote:

> Default phys-bits on Qemu is TCG_PHYS_BITS (40) which is enough
> to address 1Tb (0xff ffff ffff). On AMD platforms, if a
> ram-above-4g relocation happens and the CPU wasn't configured
> with a big enough phys-bits, warn the user. There isn't a
> catastrophic failure exactly, the guest will still boot, but
> most likely won't be able to use more than ~4G of RAM.
>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> ---
>  hw/i386/pc.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index b060aedd38f3..f8712eb8427e 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -842,6 +842,7 @@ static void relocate_4g(MachineState *machine, PCMachineState *pcms)
>      X86MachineState *x86ms = X86_MACHINE(pcms);
>      ram_addr_t device_mem_size = 0;
>      uint32_t eax, vendor[3];
> +    hwaddr maxphysaddr;
>
>      host_cpuid(0x0, 0, &eax, &vendor[0], &vendor[2], &vendor[1]);
>      if (!IS_AMD_VENDOR(vendor)) {
> @@ -858,6 +859,12 @@ static void relocate_4g(MachineState *machine, PCMachineState *pcms)
>          return;
>      }
>
> +    maxphysaddr = ((hwaddr)1 << X86_CPU(first_cpu)->phys_bits) - 1;
> +    if (maxphysaddr < AMD_ABOVE_1TB_START)

Braces around the block are required, I believe.

> +        warn_report("Relocated RAM above 4G to start at %lu "

Should use PRIu64?

> +                    "phys-bits too low (%u)",
> +                    AMD_ABOVE_1TB_START, X86_CPU(first_cpu)->phys_bits);
> +
>      x86ms->above_4g_mem_start = AMD_ABOVE_1TB_START;

And a real nit - until above_4g_mem_start is modified, the number of
phys_bits is fine, so I would have put the warning after the assignment.

>  }

dme.
-- 
Tonight I'm gonna bury that horse in the ground.