On Thu, 09 Jul 2015 09:02:38 +0200
Laszlo Ersek <lersek@xxxxxxxxxx> wrote:
> 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 <lersek@xxxxxxxxxx>
> > Signed-off-by: Bandan Das <bsd@xxxxxxxxxx>
> > ---
> > 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).
also with memory hotplug tuned on we should check if the end of
hotplug memory area is less then limit, i.e.:
pcms->hotplug_memory.base + hotplug_mem_size < 1ULL << max_phys_bits
>
> Thanks
> Laszlo
>
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