linux-kernel - Re: [PATCH Part2 v6 12/49] crypto: ccp: Add support to initialize the AMD-SP for SEV-SNP

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Date:   Tue, 25 Oct 2022 10:30:56 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Kalra, Ashish" <ashish.kalra@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
        ardb@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
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        dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com,
        peterz@...radead.org, srinivas.pandruvada@...ux.intel.com,
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        dgilbert@...hat.com, jarkko@...nel.org
Subject: Re: [PATCH Part2 v6 12/49] crypto: ccp: Add support to initialize
 the AMD-SP for SEV-SNP

On Fri, Oct 14, 2022 at 04:09:11PM -0500, Kalra, Ashish wrote:
> Yes, we need to do:
> 
> wbinvd_on_all_cpus();
> SNP_DF_FLUSH
> 
> Need to ensure all the caches are clear before launching the first guest and
> this has to be a combination of WBINVD and SNP_DF_FLUSH command.

Ok.

> > Why isn't this retval checked?
> 
> From the SNP FW ABI specs, for the SNP_SHUTDOWN command:
> 
> Firmware checks for every encryption capable ASID that the ASID is not in
> use by a guest and a DF_FLUSH is not required. If a DF_FLUSH is required,
> the firmware returns DFFLUSH_REQUIRED.
>
> Considering that SNP_SHUTDOWN command will check if DF_FLUSH was
> required and if so, and not invoked before that command, returns
> an error indicating that DFFLUSH is required.
> 
> This way, we can cleverly avoid taking the error code path for
> DF_FLUSH command here and instead let the SNP_SHUTDOWN command
> failure below indicate if DF_FLUSH command failed.
> 
> This also ensures that we always invoke SNP_SHUTDOWN command,
> irrespective of SNP_DF_FLUSH command failure as SNP_DF_FLUSH may
> actually not be required by the SHUTDOWN command.

This all sounds just silly. The proper way to do this is:

retry:
        ret = __sev_do_cmd_locked(SEV_CMD_SNP_SHUTDOWN, NULL, error);
        if (ret == DFFLUSH_REQUIRED) {
		ret = __sev_do_cmd_locked(SEV_CMD_SNP_DF_FLUSH, NULL, NULL);
		if (ret)
			"... DF_FLUSH failed...";

		goto retry;
	}

I'm assuming here the firmware is smart enough to not keep returning
DFFLUSH_REQUIRED constantly and cause an endless loop.

Thx.

-- 
Regards/Gruss,
    Boris.

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