On Mon, Aug 11, 2014 at 11:23:04PM +0900, Jungseok Lee wrote:
> On Aug 11, 2014, at 8:35 PM, Christoffer Dall wrote:
> > On Mon, Aug 11, 2014 at 12:24:35PM +0100, Richard W.M. Jones wrote:
> >> On Mon, Aug 11, 2014 at 01:20:46PM +0200, Christoffer Dall wrote:
> >>> On Sun, Aug 10, 2014 at 02:24:04PM +0100, Richard W.M. Jones wrote:
> >>>> kvm_alloc_stage2_pgd has to do an order 9 allocation, ie. 512
> >>>> contiguous pages I think.
> >>>>
> >>>> This often leads to problems running qemu when memory is relatively
> >>>> low -- eg. if you have one VM running, a healthy number of host
> >>>> applications, and perhaps "just" 4GB free; then you decide to run the
> >>>> libguestfs test suite.
> >>>>
> >>>> Any suggestions how to deal with this?
> >>>>
> >>> I'm not familiar with the libguestfs test suite, but are you saying you
> >>> have 4GB of free physical memory and when you start your first VM then
> >>> you get this error? That sounds unlikely to me.
> >>
> >> No, it runs hundreds of appliances (not all at the same time). Some
> >> fail.
> >>
> >> It seems to be a memory fragmentation issue, rather than the absolute
> >> free memory.
> >>
> > Ok, that's what I thought. You can probably hack around it by reducing
> > S2_PGD_ORDER to whatever is accessible by the VMs you wish to run (as
> > Jungseok also points out), but I'm afraid an upstream solution is
> > probably not ready before the next merge window opens, at least.
>
> In case of ARM64 KVM, it is possible to reduce S2_PGD_ORDER in the following way.
>
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -62,7 +62,28 @@
> * Align KVM with the kernel's view of physical memory. Should be
> * 40bit IPA, with PGD being 8kB aligned in the 4KB page configuration.
> */
> -#define KVM_PHYS_SHIFT PHYS_MASK_SHIFT
> +static inline int kvm_get_pa_range(void)
> +{
> + int pa_range = read_cpuid(ID_AA64MMFR0_EL1) & 0xf;
> +
> + switch (pa_range) {
> + case 0:
> + return 32;
> + case 1:
> + return 36;
> + case 2:
> + return 40;
> + case 3:
> + return 42;
> + case 4:
> + return 44;
> + case 5:
> + return 48;
> + default:
> + return -EINVAL;
> + }
> +}
> +#define KVM_PHYS_SHIFT kvm_get_pa_range()
> #define KVM_PHYS_SIZE (1UL << KVM_PHYS_SHIFT)
> #define KVM_PHYS_MASK (KVM_PHYS_SIZE - 1UL)
>
> The code puts limitation on guest's address space which is at most
> host's physical address space. For example, if host runs on Cortex-A57,
> IPA is set to 44, not 48.
>
> If this approach looks reasonable, I will post it as 3.17-rc1 comes up.
> If not, please ignore it or use it as hack.
>
Did you check what happens when handling a stage-2 translation fault due
to the input address being larger than the address space specified by
the T0SZ field?
My feeling is that this should only be included in a proper rework of
the supported guest physical address sizes.
-Christoffer
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