Re: [Qemu-devel] [PATCH] target-i386: Sanity check host processor physical address width

[Laszlo Ersek]

On 07/09/15 00:42, Bandan Das wrote:
> 
> If a Linux guest is assigned more memory than is supported
> by the host processor, the guest is unable to boot. That
> is expected, however, there's no message indicating the user
> what went wrong. This change prints a message to stderr if
> KVM has the corresponding capability.
> 
> Reported-by: Laszlo Ersek <address@hidden>
> Signed-off-by: Bandan Das <address@hidden>
> ---
>  linux-headers/linux/kvm.h | 1 +
>  target-i386/kvm.c         | 6 ++++++
>  2 files changed, 7 insertions(+)
> 
> diff --git a/linux-headers/linux/kvm.h b/linux-headers/linux/kvm.h
> index 3bac873..6afad49 100644
> --- a/linux-headers/linux/kvm.h
> +++ b/linux-headers/linux/kvm.h
> @@ -817,6 +817,7 @@ struct kvm_ppc_smmu_info {
>  #define KVM_CAP_DISABLE_QUIRKS 116
>  #define KVM_CAP_X86_SMM 117
>  #define KVM_CAP_MULTI_ADDRESS_SPACE 118
> +#define KVM_CAP_PHY_ADDR_WIDTH 119
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> diff --git a/target-i386/kvm.c b/target-i386/kvm.c
> index 066d03d..66e3448 100644
> --- a/target-i386/kvm.c
> +++ b/target-i386/kvm.c
> @@ -892,6 +892,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>      uint64_t shadow_mem;
>      int ret;
>      struct utsname utsname;
> +    int max_phys_bits;
>  
>      ret = kvm_get_supported_msrs(s);
>      if (ret < 0) {
> @@ -945,6 +946,11 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>          }
>      }
>  
> +    max_phys_bits = kvm_check_extension(s, KVM_CAP_PHY_ADDR_WIDTH);
> +    if (max_phys_bits && (1ULL << max_phys_bits) <= ram_size)
> +        fprintf(stderr, "Warning: The amount of memory assigned to the guest 
> "
> +            "is more than that supported by the host CPU(s). Guest may be 
> unstable.\n");
> +
>      if (kvm_check_extension(s, KVM_CAP_X86_SMM)) {
>          smram_machine_done.notify = register_smram_listener;
>          qemu_add_machine_init_done_notifier(&smram_machine_done);
> 

First, see my comments on the KVM patch.

Second, ram_size is not the right thing to compare. What should be
checked is whether the highest guest-physical address that maps to RAM
can be represented in the address width of the host processor (and only
if EPT is enabled, but that sub-condition belongs to the KVM patch).

Note that this is not the same as the check written in the patch. For
example, if you assume a 32-bit PCI hole with size 1 GB, then a total
guest RAM of size 63 GB will result in the highest guest-phys memory
address being 0xF_FFFF_FFFF, which just fits into 36 bits.

Correspondingly, the above code would not print the warning for

  -m $((63 * 1024 + 1))

on my laptop (which has "address sizes   : 36 bits physical, ..."), even
though such a guest would not boot for me (with EPT enabled).

Please see

http://thread.gmane.org/gmane.comp.bios.tianocore.devel/15418/focus=15447

So, "ram_size" in the controlling expression should be replaced with
"maximum_guest_ram_address" (which should be inclusive, and the <= relop
should be preserved).

Thanks
Laszlo