linux-kernel - RE: [PATCH 2/5] KVM: x86/mmu: Fine-grained check of whether a invalid & RAM PFN is MMIO

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Message-ID: <BN9PR11MB5276CABD8B3E1772932E2E3D8CE42@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Tue, 7 May 2024 08:39:27 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: "Zhao, Yan Y" <yan.y.zhao@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>,
	"alex.williamson@...hat.com" <alex.williamson@...hat.com>, "jgg@...dia.com"
	<jgg@...dia.com>
CC: "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"luto@...nel.org" <luto@...nel.org>, "peterz@...radead.org"
	<peterz@...radead.org>, "tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
	"hpa@...or.com" <hpa@...or.com>, "corbet@....net" <corbet@....net>,
	"joro@...tes.org" <joro@...tes.org>, "will@...nel.org" <will@...nel.org>,
	"robin.murphy@....com" <robin.murphy@....com>, "baolu.lu@...ux.intel.com"
	<baolu.lu@...ux.intel.com>, "Liu, Yi L" <yi.l.liu@...el.com>
Subject: RE: [PATCH 2/5] KVM: x86/mmu: Fine-grained check of whether a invalid
 & RAM PFN is MMIO

> From: Zhao, Yan Y <yan.y.zhao@...el.com>
> Sent: Tuesday, May 7, 2024 2:20 PM
> @@ -101,9 +101,21 @@ static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
>  			 */
>  			(!pat_enabled() ||
> pat_pfn_immune_to_uc_mtrr(pfn));
> 
> +	/*
> +	 * If the PFN is invalid and not RAM in raw e820 table, keep treating it
> +	 * as MMIO.
> +	 *
> +	 * If the PFN is invalid and is RAM in raw e820 table,
> +	 * - if PAT is not enabled, always treat the PFN as MMIO to avoid
> futher
> +	 *   checking of MTRRs.
> +	 * - if PAT is enabled, treat the PFN as MMIO if its PAT is UC/WC/UC-
> in
> +	 *   primary MMU.
> +	 * to prevent guest cacheable access to MMIO PFNs.
> +	 */
>  	return !e820__mapped_raw_any(pfn_to_hpa(pfn),
>  				     pfn_to_hpa(pfn + 1) - 1,
> -				     E820_TYPE_RAM);
> +				     E820_TYPE_RAM) ? true :
> +				     (!pat_enabled() ||
> pat_pfn_immune_to_uc_mtrr(pfn));

Is it for another theoretical problem in case the primary
mmu uses a non-WB type on a invalid RAM-type pfn so
you want to do additional scrutiny here?