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

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