linux-kernel - Re: KVM selftests are failing

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Date:   Thu, 15 Nov 2018 15:36:44 +0200
From:   Ahmed Soliman <ahmedsoliman@...a.vt.edu>
To:     Andrew Jones <drjones@...hat.com>,
        Anders Roxell <anders.roxell@...aro.org>
Cc:     Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: KVM selftests are failing

Hello,

On Thu, Nov 15, 2018 at 11:02 AM Andrew Jones <drjones@...hat.com> wrote:
> I think you should be able to pre-compile the selftests and save them,
> and then bisect the kernel further back than their introduction. Actually
> you may want to simplify the test case to just a mmap/madvise pair and
> see if that reproduces, since kvm selftests doesn't do much more than
> that. (Other than all the /dev/kvm ioctls - you have the right perms
> on /dev/kvm, or are running as root, right?)
yes I have /dev/kvm permissions, but it shouldn't matter at the moment.

well I stripped the kvm self tests from most stuff and ended up with
simpler code
(sorry for spaces, I am using web interface to send this email)


#include <stdio.h>
#include <sys/mman.h>
#include <string.h>
#include <errno.h>
static void *align(void *x, size_t size)
{
       size_t mask = size - 1;
       return (void *) (((size_t) x + mask) & ~mask);
}

int main(){

       int ret;
       void *mmap_start;
       void *host_mem;
       size_t length = 6291456;

       mmap_start = mmap(NULL, length, PROT_READ | PROT_WRITE,
                       MAP_PRIVATE | MAP_ANONYMOUS | 0, -1, 0);
       host_mem = align(mmap_start, 1);
       ret = madvise(host_mem, length, MADV_NOHUGEPAGE);
       printf("madvise returned: %d\nerrno: %d %s\n", ret, errno,
strerror(errno));
}

Now I couldn't find a single commit to bisect from, I checked 4.20,
4.19, 4.16, 4.10, 4.4 back to v3.18 tag
and None of the builds I attempted did work. All of the kernels returned:

madvise returned: -1
errno: 22 Invalid argument

and the strace logs looks like this:

mmap(NULL, 6291456, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,
-1, 0) = 0x7f46ea2a1000
madvise(0x7f46ea2a1000, 6291456, MADV_NOHUGEPAGE) = -1 EINVAL (Invalid argument)

For comprehension, this is done on intel core i7-4500U CPU @ 1.80GHz

As for now I will comment the madvise line and the assert when writing
my own kvm self test. I think it wouldn't cause any trouble?, If it is
not the case, please let me know.

Thanks,

-- 
Ahmed
Junior Researcher, IoT and Cyber Security lab, SmartCI, Alexandria
University, & CIS @  VMI