Re: [PATCH v10 9/9] KVM: Enable and expose KVM_MEM_PRIVATE

Chao Peng
2023-03-08T07:48:20Z

On Wed, Mar 08, 2023 at 12:13:24AM +0000, Ackerley Tng wrote:
> Chao Peng <chao.p.peng@linux.intel.com> writes:
>
> > On Sat, Jan 14, 2023 at 12:01:01AM +0000, Sean Christopherson wrote:
> > > On Fri, Dec 02, 2022, Chao Peng wrote:
> > ...
> > > Strongly prefer to use similar logic to existing code that detects wraps:
>
> > > mem->restricted_offset + mem->memory_size < mem->restricted_offset
>
> > > This is also where I'd like to add the "gfn is aligned to offset"
> > > check, though
> > > my brain is too fried to figure that out right now.
>
> > Used count_trailing_zeros() for this TODO, unsure we have other better
> > approach.
>
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index afc8c26fa652..fd34c5f7cd2f 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -56,6 +56,7 @@
> > #include <asm/processor.h>
> > #include <asm/ioctl.h>
> > #include <linux/uaccess.h>
> > +#include <linux/count_zeros.h>
>
> > #include "coalesced_mmio.h"
> > #include "async_pf.h"
> > @@ -2087,6 +2088,19 @@ static bool kvm_check_memslot_overlap(struct
> > kvm_memslots *slots, int id,
> > return false;
> > }
>
> > +/*
> > + * Return true when ALIGNMENT(offset) >= ALIGNMENT(gpa).
> > + */
> > +static bool kvm_check_rmem_offset_alignment(u64 offset, u64 gpa)
> > +{
> > + if (!offset)
> > + return true;
> > + if (!gpa)
> > + return false;
> > +
> > + return !!(count_trailing_zeros(offset) >= count_trailing_zeros(gpa));
>
> Perhaps we could do something like
>
> #define lowest_set_bit(val) (val & -val)
>
> and use
>
> return lowest_set_bit(offset) >= lowest_set_bit(gpa);

I see kernel already has fls64(), that looks what we need ;)

>
> Please help me to understand: why must ALIGNMENT(offset) >=
> ALIGNMENT(gpa)? Why is it not sufficient to have both gpa and offset be
> aligned to PAGE_SIZE?

Yes, it's sufficient. Here we just want to be conservative on the uAPI
as Sean explained this at [1]:

I would rather reject memslot if the gfn has lesser alignment than the
offset. I'm totally ok with this approach _if_ there's a use case.
Until such a use case presents itself, I would rather be conservative
from a uAPI perspective.

[1] https://lore.kernel.org/all/Y8HldeHBrw+OOZVm@google.com/

Chao
>
> > +}
> > +
> > /*
> > * Allocate some memory and give it an address in the guest physical
> > address
> > * space.
> > @@ -2128,7 +2142,8 @@ int __kvm_set_memory_region(struct kvm *kvm,
> > if (mem->flags & KVM_MEM_PRIVATE &&
> > (mem->restrictedmem_offset & (PAGE_SIZE - 1) ||
> > mem->restrictedmem_offset + mem->memory_size <
> > mem->restrictedmem_offset ||
> > - 0 /* TODO: require gfn be aligned with restricted offset */))
> > + !kvm_check_rmem_offset_alignment(mem->restrictedmem_offset,
> > + mem->guest_phys_addr)))
> > return -EINVAL;
> > if (as_id >= kvm_arch_nr_memslot_as_ids(kvm) || id >= KVM_MEM_SLOTS_NUM)
> > return -EINVAL;