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

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/07/2015 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 <lersek@redhat.com>
> Signed-off-by: Bandan Das <bsd@redhat.com>

This is not entirely correct, because it doesn't take the PCI hole into
account.

Perhaps KVM could simply hide memory above the limit (i.e. treat it as
MMIO), and the BIOS could remove RAM above the limit from the e820
memory map?

Paolo

> ---
>  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);
>