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

Chao Peng
2023-03-24T02:18:19Z

On Wed, Mar 22, 2023 at 05:41:31PM -0700, Isaku Yamahata wrote:
> On Wed, Mar 08, 2023 at 03:40:26PM +0800,
> Chao Peng <chao.p.peng@linux.intel.com> wrote:
>
> > 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));
>
> This check doesn't work expected. For example, offset = 2GB, gpa=4GB
> this check fails.

This case is expected to fail as Sean initially suggested[*]:
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.

I understand that we put tighter restriction on this but if you see such
restriction is really a big issue for real usage, instead of a
theoretical problem, then we can loosen the check here. But at that time
below code is kind of x86 specific and may need improve.

BTW, in latest code, I replaced count_trailing_zeros() with fls64():
return !!(fls64(offset) >= fls64(gpa));

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

Chao
> I come up with the following.
>
> >From ec87e25082f0497431b732702fae82c6a05071bf Mon Sep 17 00:00:00 2001
> Message-Id: <ec87e25082f0497431b732702fae82c6a05071bf.1679531995.git.isaku.yamahata@intel.com>
> From: Isaku Yamahata <isaku.yamahata@intel.com>
> Date: Wed, 22 Mar 2023 15:32:56 -0700
> Subject: [PATCH] KVM: Relax alignment check for restricted mem
>
> kvm_check_rmem_offset_alignment() only checks based on offset alignment
> and GPA alignment. However, the actual alignment for offset depends
> on architecture. For x86 case, it can be 1G, 2M or 4K. So even if
> GPA is aligned for 1G+, only 1G-alignment is required for offset.
>
> Without this patch, gpa=4G, offset=2G results in failure of memory slot
> creation.
>
> Fixes: edc8814b2c77 ("KVM: Require gfn be aligned with restricted offset")
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> ---
> arch/x86/include/asm/kvm_host.h | 15 +++++++++++++++
> virt/kvm/kvm_main.c | 9 ++++++++-
> 2 files changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 88e11dd3afde..03af44650f24 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -16,6 +16,7 @@
> #include <linux/irq_work.h>
> #include <linux/irq.h>
> #include <linux/workqueue.h>
> +#include <linux/count_zeros.h>
>
> #include <linux/kvm.h>
> #include <linux/kvm_para.h>
> @@ -143,6 +144,20 @@
> #define KVM_HPAGE_MASK(x) (~(KVM_HPAGE_SIZE(x) - 1))
> #define KVM_PAGES_PER_HPAGE(x) (KVM_HPAGE_SIZE(x) / PAGE_SIZE)
>
> +#define kvm_arch_required_alignment kvm_arch_required_alignment
> +static inline int kvm_arch_required_alignment(u64 gpa)
> +{
> + int zeros = count_trailing_zeros(gpa);
> +
> + WARN_ON_ONCE(!PAGE_ALIGNED(gpa));
> + if (zeros >= KVM_HPAGE_SHIFT(PG_LEVEL_1G))
> + return KVM_HPAGE_SHIFT(PG_LEVEL_1G);
> + else if (zeros >= KVM_HPAGE_SHIFT(PG_LEVEL_2M))
> + return KVM_HPAGE_SHIFT(PG_LEVEL_2M);
> +
> + return PAGE_SHIFT;
> +}
> +
> #define KVM_MEMSLOT_PAGES_TO_MMU_PAGES_RATIO 50
> #define KVM_MIN_ALLOC_MMU_PAGES 64UL
> #define KVM_MMU_HASH_SHIFT 12
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index c9c4eef457b0..f4ff96171d24 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2113,6 +2113,13 @@ static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
> return false;
> }
>
> +#ifndef kvm_arch_required_alignment
> +__weak int kvm_arch_required_alignment(u64 gpa)
> +{
> + return PAGE_SHIFT
> +}
> +#endif
> +
> /*
> * Return true when ALIGNMENT(offset) >= ALIGNMENT(gpa).
> */
> @@ -2123,7 +2130,7 @@ static bool kvm_check_rmem_offset_alignment(u64 offset, u64 gpa)
> if (!gpa)
> return false;
>
> - return !!(count_trailing_zeros(offset) >= count_trailing_zeros(gpa));
> + return !!(count_trailing_zeros(offset) >= kvm_arch_required_alignment(gpa));
> }
>
> /*
> --
> 2.25.1
>
>
>
> --
> Isaku Yamahata <isaku.yamahata@gmail.com>