linux-kernel - Re: [PATCH 10/22] kvm: mmu: Add TDP MMU PF handler

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Date:   Wed, 30 Sep 2020 19:37:57 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 10/22] kvm: mmu: Add TDP MMU PF handler

On 30/09/20 18:37, Sean Christopherson wrote:
>> +	ret = page_fault_handle_target_level(vcpu, write, map_writable,
>> +					     as_id, &iter, pfn, prefault);
>> +
>> +	/* If emulating, flush this vcpu's TLB. */
> Why?  It's obvious _what_ the code is doing, the comment should explain _why_.
> 
>> +	if (ret == RET_PF_EMULATE)
>> +		kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
>> +
>> +	return ret;
>> +}

In particular it seems to be only needed in this case...

+	/*
+	 * If the page fault was caused by a write but the page is write
+	 * protected, emulation is needed. If the emulation was skipped,
+	 * the vCPU would have the same fault again.
+	 */
+	if ((make_spte_ret & SET_SPTE_WRITE_PROTECTED_PT) && write)
+		ret = RET_PF_EMULATE;
+

... corresponding to this code in mmu.c

        if (set_spte_ret & SET_SPTE_WRITE_PROTECTED_PT) {
                if (write_fault)
                        ret = RET_PF_EMULATE;
                kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
        }

So it should indeed be better to make the code in
page_fault_handle_target_level look the same as mmu/mmu.c.

Paolo