linux-kernel - Re: [PATCH v7 05/12] KVM: guest_memfd: Add flag to remove from direct map

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <356ece73-83ff-4c0f-a3fb-4b24df7953b3@amazon.com>
Date: Fri, 5 Dec 2025 17:23:22 +0000
From: Nikita Kalyazin <kalyazin@...zon.com>
To: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
CC: "pbonzini@...hat.com" <pbonzini@...hat.com>, "corbet@....net"
	<corbet@....net>, "maz@...nel.org" <maz@...nel.org>, "oliver.upton@...ux.dev"
	<oliver.upton@...ux.dev>, "joey.gouly@....com" <joey.gouly@....com>,
	"suzuki.poulose@....com" <suzuki.poulose@....com>, "yuzenghui@...wei.com"
	<yuzenghui@...wei.com>, "catalin.marinas@....com" <catalin.marinas@....com>,
	"will@...nel.org" <will@...nel.org>, "tglx@...utronix.de"
	<tglx@...utronix.de>, "mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de"
	<bp@...en8.de>, "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
	"luto@...nel.org" <luto@...nel.org>, "peterz@...radead.org"
	<peterz@...radead.org>, "willy@...radead.org" <willy@...radead.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"lorenzo.stoakes@...cle.com" <lorenzo.stoakes@...cle.com>,
	"Liam.Howlett@...cle.com" <Liam.Howlett@...cle.com>, "vbabka@...e.cz"
	<vbabka@...e.cz>, "rppt@...nel.org" <rppt@...nel.org>, "surenb@...gle.com"
	<surenb@...gle.com>, "mhocko@...e.com" <mhocko@...e.com>, "song@...nel.org"
	<song@...nel.org>, "jolsa@...nel.org" <jolsa@...nel.org>, "ast@...nel.org"
	<ast@...nel.org>, "daniel@...earbox.net" <daniel@...earbox.net>,
	"andrii@...nel.org" <andrii@...nel.org>, "martin.lau@...ux.dev"
	<martin.lau@...ux.dev>, "eddyz87@...il.com" <eddyz87@...il.com>,
	"yonghong.song@...ux.dev" <yonghong.song@...ux.dev>,
	"john.fastabend@...il.com" <john.fastabend@...il.com>, "kpsingh@...nel.org"
	<kpsingh@...nel.org>, "sdf@...ichev.me" <sdf@...ichev.me>,
	"haoluo@...gle.com" <haoluo@...gle.com>, "jgg@...pe.ca" <jgg@...pe.ca>,
	"jhubbard@...dia.com" <jhubbard@...dia.com>, "peterx@...hat.com"
	<peterx@...hat.com>, "jannh@...gle.com" <jannh@...gle.com>,
	"pfalcato@...e.de" <pfalcato@...e.de>, "shuah@...nel.org" <shuah@...nel.org>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "kvmarm@...ts.linux.dev"
	<kvmarm@...ts.linux.dev>, "linux-fsdevel@...r.kernel.org"
	<linux-fsdevel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	"bpf@...r.kernel.org" <bpf@...r.kernel.org>,
	"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>, "Cali,
 Marco" <xmarcalx@...zon.co.uk>, "Kalyazin, Nikita" <kalyazin@...zon.co.uk>,
	"Thomson, Jack" <jackabt@...zon.co.uk>, "derekmn@...zon.co.uk"
	<derekmn@...zon.co.uk>, "tabba@...gle.com" <tabba@...gle.com>,
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "david@...nel.org"
	<david@...nel.org>, "patrick.roy@...ux.dev" <patrick.roy@...ux.dev>
Subject: Re: [PATCH v7 05/12] KVM: guest_memfd: Add flag to remove from direct
 map



On 03/11/2025 07:57, Aneesh Kumar K.V wrote:
> "Roy, Patrick" <roypat@...zon.co.uk> writes:
> 
> ....
> 
>> +static int kvm_gmem_folio_zap_direct_map(struct folio *folio)
>> +{
>> +     if (kvm_gmem_folio_no_direct_map(folio))
>> +             return 0;
>> +
>> +     int r = set_direct_map_valid_noflush(folio_page(folio, 0), folio_nr_pages(folio),
>> +                                      false);
>> +
>> +     if (!r) {
>> +             unsigned long addr = (unsigned long) folio_address(folio);
>> +             folio->private = (void *) ((u64) folio->private & KVM_GMEM_FOLIO_NO_DIRECT_MAP);
>> +             flush_tlb_kernel_range(addr, addr + folio_size(folio));
>> +     }
>> +
>> +     return r;
>> +}
> 
> These 'noflush' functions are actually doing flush_tlb_kernel
> 
> [-]  ∘ flush_tlb_kernel_range
>   |-[-]  ← __change_memory_common
>   |  `-[-]  ← set_memory_valid
>   |     `-   ← set_direct_map_valid_noflush

Hi Aneesh,

Thanks for pointing at that.  I ran internal tests and it appears that 
the second flush_tlb_kernel_range() call does add a latency similar to 
the one coming from the first call, even though it intuitively should be 
a no-op.  I have to admit that I am not aware of a safe way to avoid the 
second flushing on ARM while keeping the guest_memfd code arch-agnostic. 
  Perhaps I should seek Will's counsel for it.  Nevertheless, I don't 
think there is a concern from the functional point of view.

Nikita

> 
> -aneesh