linux-kernel - Re: [PATCH 7/7] KVM: selftests: Add "delete" testcase to set_memory_region

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Message-ID: <20200323214317.GV28711@linux.intel.com>
Date:   Mon, 23 Mar 2020 14:43:18 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Qian Cai <cai@....pw>
Subject: Re: [PATCH 7/7] KVM: selftests: Add "delete" testcase to
 set_memory_region_test

On Mon, Mar 23, 2020 at 03:06:36PM -0400, Peter Xu wrote:
> On Fri, Mar 20, 2020 at 01:55:46PM -0700, Sean Christopherson wrote:
> > +	/*
> > +	 * Spin until the memory region is moved to a misaligned address.  This
> > +	 * may or may not trigger MMIO, as the window where the memslot is
> > +	 * invalid is quite small.
> > +	 */
> > +	val = guest_spin_on_val(0);
> > +	GUEST_ASSERT(val == 1 || val == MMIO_VAL);
> > +
> > +	/* Spin until the memory region is realigned. */
> > +	GUEST_ASSERT(guest_spin_on_val(MMIO_VAL) == 1);
> 
> IIUC ideally we should do GUEST_SYNC() after each GUEST_ASSERT() to
> make sure the two threads are in sync.  Otherwise e.g. there's no
> guarantee that the main thread won't run too fast to quickly remove
> the memslot and re-add it back before the guest_spin_on_val() starts
> above, then the assert could trigger when it reads the value as zero.

Hrm, I was thinking ucall wasn't available across pthreads, but it's just
dumped into a global variable.  I'll rework this to replace the udelay()
hacks with proper synchronization.