linux-kernel - Re: [PATCH v3 7/8] KVM: x86/mmu: Protect rmaps independently with SRCU

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Date:   Fri, 7 May 2021 10:42:19 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Kai Huang <kai.huang@...el.com>,
        Keqian Zhu <zhukeqian1@...wei.com>
Subject: Re: [PATCH v3 7/8] KVM: x86/mmu: Protect rmaps independently with
 SRCU

On 06/05/21 20:42, Ben Gardon wrote:
> In preparation for lazily allocating the rmaps when the TDP MMU is in
> use, protect the rmaps with SRCU. Unfortunately, this requires
> propagating a pointer to struct kvm around to several functions.

Thinking more about it, this is not needed because all reads of the rmap 
array are guarded by the load-acquire of kvm->arch.memslots_have_rmaps. 
  That is, the pattern is always

	if (!load-acquire(memslot_have_rmaps))
		return;
	... = __gfn_to_rmap(...)

				slots->arch.rmap[x] = ...
				store-release(memslot_have_rmaps, true)

where the load-acquire/store-release have the same role that 
srcu_dereference/rcu_assign_pointer had before this patch.

We also know that any read that misses the check has the potential for a 
NULL pointer dereference, so it *has* to be like that.

That said, srcu_dereference has zero cost unless debugging options are 
enabled, and it *is* true that the rmap can disappear if kvm->srcu is 
not held, so I lean towards keeping this change and just changing the 
commit message like this:

---------
Currently, rmaps are always allocated and published together with a new 
memslot, so the srcu_dereference for the memslots array already ensures 
that the memory pointed to by slots->arch.rmap is zero at the time 
slots->arch.rmap.  However, they still need to be accessed in an SRCU 
read-side critical section, as the whole memslot can be deleted outside 
SRCU.
--------

Thanks,

Paolo

> 
> Suggested-by: Paolo Bonzini<pbonzini@...hat.com>
> Signed-off-by: Ben Gardon<bgardon@...gle.com>
> ---
>   arch/x86/kvm/mmu/mmu.c | 57 +++++++++++++++++++++++++-----------------
>   arch/x86/kvm/x86.c     |  6 ++---
>   2 files changed, 37 insertions(+), 26 deletions(-)