linux-kernel - Re: [RFC PATCH V3 04/11] HV: Add Write/Read MSR registers via ghcb

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Message-ID: <YMC4JdtYO+eLDKh5@8bytes.org>
Date:   Wed, 9 Jun 2021 14:46:29 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Tianyu Lan <ltykernel@...il.com>
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        wei.liu@...nel.org, decui@...rosoft.com, tglx@...utronix.de,
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        hannes@...xchg.org, cai@....pw, krish.sadhukhan@...cle.com,
        saravanand@...com, Tianyu.Lan@...rosoft.com,
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Subject: Re: [RFC PATCH V3 04/11] HV: Add Write/Read MSR registers via ghcb

On Sun, May 30, 2021 at 11:06:21AM -0400, Tianyu Lan wrote:
> +void hv_ghcb_msr_write(u64 msr, u64 value)
> +{
> +	union hv_ghcb *hv_ghcb;
> +	void **ghcb_base;
> +	unsigned long flags;
> +
> +	if (!ms_hyperv.ghcb_base)
> +		return;
> +
> +	local_irq_save(flags);
> +	ghcb_base = (void **)this_cpu_ptr(ms_hyperv.ghcb_base);
> +	hv_ghcb = (union hv_ghcb *)*ghcb_base;
> +	if (!hv_ghcb) {
> +		local_irq_restore(flags);
> +		return;
> +	}
> +
> +	memset(hv_ghcb, 0x00, HV_HYP_PAGE_SIZE);
> +
> +	hv_ghcb->ghcb.protocol_version = 1;
> +	hv_ghcb->ghcb.ghcb_usage = 0;
> +
> +	ghcb_set_sw_exit_code(&hv_ghcb->ghcb, SVM_EXIT_MSR);
> +	ghcb_set_rcx(&hv_ghcb->ghcb, msr);
> +	ghcb_set_rax(&hv_ghcb->ghcb, lower_32_bits(value));
> +	ghcb_set_rdx(&hv_ghcb->ghcb, value >> 32);
> +	ghcb_set_sw_exit_info_1(&hv_ghcb->ghcb, 1);
> +	ghcb_set_sw_exit_info_2(&hv_ghcb->ghcb, 0);
> +
> +	VMGEXIT();

This is not safe to use from NMI context. You need at least some
checking or WARN_ON/assertion/whatever to catch cases where this is
violated. Otherwise it will result in some hard to debug bug reports.

Regards,

	Joerg