linux-kernel - Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot

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Message-ID: <20230609132301.uvvp27yr5kpenl6f@box.shutemov.name>
Date:   Fri, 9 Jun 2023 16:23:01 +0300
From:   kirill.shutemov@...ux.intel.com
To:     Kai Huang <kai.huang@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, dave.hansen@...el.com, tony.luck@...el.com,
        peterz@...radead.org, tglx@...utronix.de, seanjc@...gle.com,
        pbonzini@...hat.com, david@...hat.com, dan.j.williams@...el.com,
        rafael.j.wysocki@...el.com, ying.huang@...el.com,
        reinette.chatre@...el.com, len.brown@...el.com, ak@...ux.intel.com,
        isaku.yamahata@...el.com, chao.gao@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, bagasdotme@...il.com,
        sagis@...gle.com, imammedo@...hat.com
Subject: Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private
 memory during kexec() and reboot

On Mon, Jun 05, 2023 at 02:27:31AM +1200, Kai Huang wrote:
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index 8ff07256a515..0aa413b712e8 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -587,6 +587,14 @@ static int tdmr_set_up_pamt(struct tdmr_info *tdmr,
>  		tdmr_pamt_base += pamt_size[pgsz];
>  	}
>  
> +	/*
> +	 * tdx_memory_shutdown() also reads TDMR's PAMT during
> +	 * kexec() or reboot, which could happen at anytime, even
> +	 * during this particular code.  Make sure pamt_4k_base
> +	 * is firstly set otherwise tdx_memory_shutdown() may
> +	 * get an invalid PAMT base when it sees a valid number
> +	 * of PAMT pages.
> +	 */

Hmm? What prevents compiler from messing this up. It can reorder as it
wishes, no?

Maybe add a proper locking? Anything that prevent preemption would do,
right?

>  	tdmr->pamt_4k_base = pamt_base[TDX_PS_4K];
>  	tdmr->pamt_4k_size = pamt_size[TDX_PS_4K];
>  	tdmr->pamt_2m_base = pamt_base[TDX_PS_2M];

-- 
  Kiryl Shutsemau / Kirill A. Shutemov