linux-kernel - Re: [PATCH v7 6/9] vmx: spp: Set up SPP paging table at vmentry/vmexit

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Message-ID: <7cdcd2b2-ced8-4c08-82c7-b3a25ed8bb15@redhat.com>
Date:   Thu, 21 Nov 2019 17:08:07 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        jmattson@...gle.com, sean.j.christopherson@...el.com,
        yu.c.zhang@...ux.intel.com, alazar@...defender.com,
        edwin.zhai@...el.com
Subject: Re: [PATCH v7 6/9] vmx: spp: Set up SPP paging table at
 vmentry/vmexit

On 21/11/19 16:22, Yang Weijiang wrote:
> On Thu, Nov 21, 2019 at 11:18:48AM +0100, Paolo Bonzini wrote:
>> On 19/11/19 09:49, Yang Weijiang wrote:
>>> +			if (spte & PT_SPP_MASK) {
>>> +				fault_handled = true;
>>> +				vcpu->run->exit_reason = KVM_EXIT_SPP;
>>> +				vcpu->run->spp.addr = gva;
>>> +				kvm_skip_emulated_instruction(vcpu);
>>
>> Do you really want to skip the current instruction?  Who will do the write?
>>
> If the destination memory is SPP protected, the target memory is
> expected unchanged on a "write op" in guest, so would like to skip current 
> instruction.

This is how you are expecting SPP to be used, but another possibility is
to unprotect and reenter the guest.  In this case
kvm_skip_emulated_instruction would be wrong (and once this decision is
made, it would be very, very hard to change it).

However, you clearly need a way to skip the instruction, and for that
you could store the current instruction length in vcpu->run->spp.  Then
userspace can adjust RIP manually if desired.

Thanks,

Paolo