linux-kernel - Re: [PATCH] kvm: selftests: Fix cut-off of addr

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Date:   Thu, 14 Apr 2022 20:30:04 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH] kvm: selftests: Fix cut-off of addr_gva2gpa lookup

On Thu, Apr 14, 2022 at 03:01:04PM -0700, Jim Mattson wrote:
> On Thu, Apr 14, 2022 at 2:36 PM Peter Xu <peterx@...hat.com> wrote:
> >
> > On Thu, Apr 14, 2022 at 04:14:22PM +0200, Paolo Bonzini wrote:
> > > On 4/14/22 15:56, Sean Christopherson wrote:
> > > > > - return (pte[index[0]].pfn * vm->page_size) + (gva & 0xfffu);
> > > > > + return ((vm_paddr_t)pte[index[0]].pfn * vm->page_size) + (gva & 0xfffu);
> > > > This is but one of many paths that can get burned by pfn being 40 bits.  The
> > > > most backport friendly fix is probably to add a pfn=>gpa helper and use that to
> > > > place the myriad "pfn * vm->page_size" instances.
> > > >
> > > > For a true long term solution, my vote is to do away with the bit field struct
> > > > and use #define'd masks and whatnot.
> > >
> > > Yes, bitfields larger than 32 bits are a mess.
> >
> > It's very interesting to know this..
> 
> I don't think the undefined behavior is restricted to extended
> bit-fields. Even for regular bit-fields, the C99 spec says, "A
> bit-field shall have a type that is a qualified or unqualified version
> of _Bool, signed
> int, unsigned int, or some other implementation-defined type." One
> might assume that even the permissive final clause refers to
> fundamental language types, but I suppose "n-bit integer" meets the
> strict definition of a "type,"
> for arbitrary values of n.

Fair enough.

I just noticed it actually make sense to have such a behavior, because in
the case of A*B where A is the bitfield (<32 bits) and when B is an int
(=32bits, page_size in the test case or a default constant value which will
also be treated as int/uint).

Then it's simply extending the smaller field into the same size as the
bigger one, as 40bits*32bits goes into a 40bits output which needs some
proper masking if calculated with RAX, while a e.g. 20bits*32bits goes into
32bits, in which case no further masking needed.

Thanks,

-- 
Peter Xu