Re: [PATCH v7 10/45] x86/sev: Add support for hypervisor feature VMGEXIT

[Borislav Petkov]

On Wed, Nov 10, 2021 at 04:06:56PM -0600, Brijesh Singh wrote:
> +/*
> + * The hypervisor features are available from GHCB version 2 onward.
> + */
> +static bool get_hv_features(void)
> +{
> +	u64 val;
> +
> +	sev_hv_features = 0;
> +
> +	if (ghcb_version < 2)
> +		return false;
> +
> +	sev_es_wr_ghcb_msr(GHCB_MSR_HV_FT_REQ);
> +	VMGEXIT();
> +
> +	val = sev_es_rd_ghcb_msr();
> +	if (GHCB_RESP_CODE(val) != GHCB_MSR_HV_FT_RESP)
> +		return false;
> +
> +	sev_hv_features = GHCB_MSR_HV_FT_RESP_VAL(val);
> +
> +	return true;
> +}

I still don't like this.

This is more of that run-me-in-the-exception-handler thing while this is
purely feature detection stuff which needs to be done exactly once on
init.

IOW, that stanza

        if (!sev_es_negotiate_protocol())
                sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_PROT_UNSUPPORTED);

should be called once in sev_enable() for the decompressor kernel and
once in sev_es_init_vc_handling() for kernel proper.

Then you don't need to do any of that sev_hv_features = 0 thing but
detect them exactly once and query them as much as you can.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette