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

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?

> +}
> +
> /*
> * 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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