linux-kernel - Re: [PATCH kernel v3] x86/compressed/64: reduce #VC nesting for intercepted CPUID for SEV-SNP guest

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Message-ID: <ZRvLp11IJqvTiOst@gmail.com>
Date:   Tue, 3 Oct 2023 10:07:03 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Alexey Kardashevskiy <aik@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH kernel v3] x86/compressed/64: reduce #VC nesting for
 intercepted CPUID for SEV-SNP guest


* Alexey Kardashevskiy <aik@....com> wrote:

> For certain intercepts an SNP guest uses the GHCB protocol to talk to
> the hypervisor from the #VC handler. The protocol requires a shared page so

s/requires a shared page so there is
 /requires a shared page, so there is

> there is one per vCPU. In case NMI arrives in a middle of #VC or the NMI
> handler triggers a #VC, there is another "backup" GHCB page which stores
> the content of the first one while SVM_VMGEXIT_NMI_COMPLETE is sent.
> The vc_raw_handle_exception() handler manages main and backup GHCB pages
> via __sev_get_ghcb/__sev_put_ghcb.
> 
> This works fine for #VC and occasional NMIs but not so fine when the #VC
> handler causes intercept + another #VC. If NMI arrives during
> the second #VC, there are no more pages for SVM_VMGEXIT_NMI_COMPLETE.
> The problem place is the #VC CPUID handler which reads an MSR which
> triggers another #VC and if "perf" was running, panic happens:
> 
> Kernel panic - not syncing: Unable to handle #VC exception! GHCB and Backup GHCB are already in use
> 
> Add a helper similar to native_read_msr_safe() for making a direct hypercall
> in the SEV-ES environment. Use the new helper instead of the raw "rdmsr" to

s/"rdmsr"
 /RDMSR

> avoid the extra #VC event.
> 
> Fixes: ee0bfa08a345 ("x86/compressed/64: Add support for SEV-SNP CPUID table in #VC handlers")
> Signed-off-by: Alexey Kardashevskiy <aik@....com>

LGTM.

Reviewed-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo