linux-kernel - Re: [PATCH v19 038/130] KVM: TDX: create/destroy VM structure

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Date: Mon, 25 Mar 2024 14:48:49 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: isaku.yamahata@...el.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
	Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
	Sean Christopherson <seanjc@...gle.com>,
	Sagi Shahar <sagis@...gle.com>, Kai Huang <kai.huang@...el.com>,
	chen.bo@...el.com, hang.yuan@...el.com, tina.zhang@...el.com,
	Sean Christopherson <sean.j.christopherson@...el.com>,
	isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH v19 038/130] KVM: TDX: create/destroy VM structure

On Mon, Mar 25, 2024 at 05:58:47PM +0800,
Binbin Wu <binbin.wu@...ux.intel.com> wrote:

> > +static void tdx_clear_page(unsigned long page_pa)
> > +{
> > +	const void *zero_page = (const void *) __va(page_to_phys(ZERO_PAGE(0)));
> > +	void *page = __va(page_pa);
> > +	unsigned long i;
> > +
> > +	/*
> > +	 * When re-assign one page from old keyid to a new keyid, MOVDIR64B is
> > +	 * required to clear/write the page with new keyid to prevent integrity
> > +	 * error when read on the page with new keyid.
> > +	 *
> > +	 * clflush doesn't flush cache with HKID set.  The cache line could be
> > +	 * poisoned (even without MKTME-i), clear the poison bit.
> > +	 */
> > +	for (i = 0; i < PAGE_SIZE; i += 64)
> > +		movdir64b(page + i, zero_page);
> > +	/*
> > +	 * MOVDIR64B store uses WC buffer.  Prevent following memory reads
> > +	 * from seeing potentially poisoned cache.
> > +	 */
> > +	__mb();
> 
> Is __wmb() sufficient for this case?

I don't think so because sfence is for other store. Here we care other load.

> > +
> > +static int tdx_do_tdh_mng_key_config(void *param)
> > +{
> > +	hpa_t *tdr_p = param;
> > +	u64 err;
> > +
> > +	do {
> > +		err = tdh_mng_key_config(*tdr_p);
> > +
> > +		/*
> > +		 * If it failed to generate a random key, retry it because this
> > +		 * is typically caused by an entropy error of the CPU's random
> 
> Here you say "typically", is there other cause and is it safe to loop on
> retry?


No as long as I know. the TDX module returns KEY_GENERATION_FAILED only when
rdrnd (or equivalent) failed.  But I don't know the future.

Let's delete "tyepically" because it seems confusing.
-- 
Isaku Yamahata <isaku.yamahata@...el.com>