linux-kernel - Re: [PATCH 1/2] KVM: introduce __vmx_flush

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Date:	Fri, 25 Sep 2015 10:01:53 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Bandan Das <bsd@...hat.com>, Wanpeng Li <wanpeng.li@...mail.com>
Cc:	Jan Kiszka <jan.kiszka@...mens.com>,
	Wincy Van <fanwenyi0529@...il.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: introduce __vmx_flush_tlb to handle specific
 vpid

On 24/09/2015 18:12, Bandan Das wrote:
> Not sure myself what's the right thing to do but this may be undesirable
> in a nested environment. Assuming the processor supports global invalidation
> only, this seems like a easy way for the nested guest to invalidate *all*
> mappings - even the L1 specific mappings.

It's not a great thing but it's already what happens if you do a global
INVEPT (it calls vmx_flush_tlb, which results in a global INVVPID if the
single-context variant is not supported).

Even without nested virt a single guest could slow down all other guests
just by triggering frequent TLB flushes (e.g. by moving around a ROM BAR
thousands of times per second).

It would help to know _which_ processors actually don't support
single-context INVVPIDs...

Paolo
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