linux-kernel - Re: [PATCH 1/1] iommu/amd: Preserve default DTE fields when updating Host Page Table Root

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Message-ID: <04eb9e26-27c9-4443-b87b-9b44e8447c52@amd.com>
Date: Tue, 7 Jan 2025 19:06:42 +0530
From: "Arun Kodilkar, Sairaj" <sarunkod@....com>
To: Vasant Hegde <vasant.hegde@....com>, Alejandro Jimenez
	<alejandro.j.jimenez@...cle.com>, <iommu@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>
CC: <joro@...tes.org>, <suravee.suthikulpanit@....com>, <will@...nel.org>,
	<jgg@...dia.com>, <joao.m.martins@...cle.com>, <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH 1/1] iommu/amd: Preserve default DTE fields when updating
 Host Page Table Root

On 1/7/2025 5:45 PM, Vasant Hegde wrote:
Hi vasant,
> 
> On 1/7/2025 12:44 AM, Alejandro Jimenez wrote:
>> When updating the page table root field on the DTE, avoid overwriting any
>> bits that are already set. The earlier call to make_clear_dte() writes
>> default values that all DTEs must have set (currently DTE[V]), and those
>> must be preserved.
>>
>> Currently this doesn't cause problems since the page table root update is
>> the first field that is set after make_clear_dte() is called, and
>> DTE_FLAG_V is set again later along with the permission bits (IR/IW).
>> Remove this redundant assignment too.
> 
> Good catch!
> 
>>> Fixes: fd5dff9de4be ("iommu/amd: Modify set_dte_entry() to use 256-bit DTE
> helpers")
>> Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@...cle.com>
>> ---
>>   drivers/iommu/amd/iommu.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
>> index 20177e18eb0d..a8c17b602c95 100644
>> --- a/drivers/iommu/amd/iommu.c
>> +++ b/drivers/iommu/amd/iommu.c
>> @@ -2052,12 +2052,12 @@ static void set_dte_entry(struct amd_iommu *iommu,
>>   	make_clear_dte(dev_data, dte, &new);
> 
> This fix is fine. But with all changes we have done in this code does, do we
> really need make_clear_dte()?
> 
> How about removing `make_clear_dte()` and moving DTE_FLAG_V to
> write_dte_lower128() ?
> 

I dont think moving DTE_FLAG_V to write_dte_lower128() is a good
idea, because the order in which the function update_dte256()
assigns upper and lower 128 bits depends on this flag.

Regards
Sairaj Kodilkar

> -Vasant
> 
> 
>