linux-kernel - Re: [RFC PATCH V6 01/14] x86/sev: Add a #HV exception handler

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Date:   Tue, 30 May 2023 14:16:55 +0200
From:   "Gupta, Pankaj" <pankaj.gupta@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Tianyu Lan <ltykernel@...il.com>
Cc:     luto@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, seanjc@...gle.com, pbonzini@...hat.com,
        jgross@...e.com, tiala@...rosoft.com, kirill@...temov.name,
        jiangshan.ljs@...group.com, ashish.kalra@....com,
        srutherford@...gle.com, akpm@...ux-foundation.org,
        anshuman.khandual@....com, pawan.kumar.gupta@...ux.intel.com,
        adrian.hunter@...el.com, daniel.sneddon@...ux.intel.com,
        alexander.shishkin@...ux.intel.com, sandipan.das@....com,
        ray.huang@....com, brijesh.singh@....com, michael.roth@....com,
        thomas.lendacky@....com, venu.busireddy@...cle.com,
        sterritt@...gle.com, tony.luck@...el.com, samitolvanen@...gle.com,
        fenghua.yu@...el.com, pangupta@....com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH V6 01/14] x86/sev: Add a #HV exception handler


>> Add a #HV exception handler that uses IST stack.
>>
> 
> Urgh.. that is entirely insufficient. Like it doesn't even begin to
> start to cover things.
> 
> The whole existing VC IST stack abuse is already a nightmare and you're
> duplicating that.. without any explanation for why this would be needed
> and how it is correct.
> 
> Please try again.

#HV handler handles both #NMI & #MCE in the guest and nested #HV is 
never raised by the hypervisor. Next #HV exception is only raised by the 
hypervisor when Guest acknowledges the pending #HV exception by clearing 
"NoFurtherSignal” bit in the doorbell page.

There is still protection (please see hv_switch_off_ist()) to gracefully 
exit the guest if by any chance a malicious hypervisor sends nested #HV. 
This saves with most of the nested IST stack pitfalls with #NMI & #MCE, 
also #DB is handled in noinstr code 
block(exc_vmm_communication()->vc_is_db {...}) hence avoid any recursive 
#DBs.

Do you see anything else needs to be handled in #HV IST handling?

Thanks,
Pankaj