linux-kernel - Re: [PATCH 0/2] Fix for a very old KVM bug in the segment cache

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Message-ID: <35aed712-d435-4660-a40a-ace7858218c4@redhat.com>
Date: Sat, 13 Jul 2024 12:22:29 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc: Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
 x86@...nel.org, linux-kernel@...r.kernel.org,
 Sean Christopherson <seanjc@...gle.com>, Ingo Molnar <mingo@...hat.com>,
 "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 0/2] Fix for a very old KVM bug in the segment cache

On 7/13/24 03:38, Maxim Levitsky wrote:
> 1. Getting rid of the segment cache. I am not sure how much it helps
> these days - this code is very old.
> 
> 2. Using a read/write lock - IMHO the cleanest solution but might
> also affect performance.

A read/write lock would cause a deadlock between the writer and the 
sched_out callback, since they run on the same CPU.

I think the root cause of the issue is that clearing the cache should be 
done _after_ the writes (and should have a barrier() at the beginning, 
if only for cleanliness).  So your patch 1 should leave the clearing of 
vmx->segment_cache.bitmask where it was.

However, that would still leave an assumption: that it's okay that a 
sched_out during vmx_vcpu_reset() (or other functions that write segment 
data in the VMCS) accesses stale data, as long as the stale data is not 
used after vmx_vcpu_reset() returns.  Your patch is a safer approach, 
but maybe wrap preempt_disable()/preempt_enable() with

	vmx_invalidate_segment_cache_start() {
		preempt_disable();
	}
	vmx_invalidate_segment_cache_end() {
		vmx->segment_cache.bitmask = 0;
		preempt_enable();
	}

Paolo