From: Catalin Marinas <catalin.marinas@arm•com>
To: Charlie Jenkins <charlie@rivosinc•com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle•com>,
Arnd Bergmann <arnd@arndb•de>, guoren <guoren@kernel•org>,
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Subject: Re: [PATCH RFC v3 1/2] mm: Add personality flag to limit address to 47 bits
Date: Wed, 11 Sep 2024 19:21:27 +0100 [thread overview]
Message-ID: <ZuHfp0_tAQhaymdy@arm.com> (raw)
In-Reply-To: <ZuDoExckq21fePoe@ghost>
On Tue, Sep 10, 2024 at 05:45:07PM -0700, Charlie Jenkins wrote:
> On Tue, Sep 10, 2024 at 03:08:14PM -0400, Liam R. Howlett wrote:
> > * Catalin Marinas <catalin.marinas@arm•com> [240906 07:44]:
> > > On Fri, Sep 06, 2024 at 09:55:42AM +0000, Arnd Bergmann wrote:
> > > > On Fri, Sep 6, 2024, at 09:14, Guo Ren wrote:
> > > > > On Fri, Sep 6, 2024 at 3:18 PM Arnd Bergmann <arnd@arndb•de> wrote:
> > > > >> It's also unclear to me how we want this flag to interact with
> > > > >> the existing logic in arch_get_mmap_end(), which attempts to
> > > > >> limit the default mapping to a 47-bit address space already.
> > > > >
> > > > > To optimize RISC-V progress, I recommend:
> > > > >
> > > > > Step 1: Approve the patch.
> > > > > Step 2: Update Go and OpenJDK's RISC-V backend to utilize it.
> > > > > Step 3: Wait approximately several iterations for Go & OpenJDK
> > > > > Step 4: Remove the 47-bit constraint in arch_get_mmap_end()
Point 4 is an ABI change. What guarantees that there isn't still
software out there that relies on the old behaviour?
> > > > I really want to first see a plausible explanation about why
> > > > RISC-V can't just implement this using a 47-bit DEFAULT_MAP_WINDOW
> > > > like all the other major architectures (x86, arm64, powerpc64),
> > >
> > > FWIW arm64 actually limits DEFAULT_MAP_WINDOW to 48-bit in the default
> > > configuration. We end up with a 47-bit with 16K pages but for a
> > > different reason that has to do with LPA2 support (I doubt we need this
> > > for the user mapping but we need to untangle some of the macros there;
> > > that's for a separate discussion).
> > >
> > > That said, we haven't encountered any user space problems with a 48-bit
> > > DEFAULT_MAP_WINDOW. So I also think RISC-V should follow a similar
> > > approach (47 or 48 bit default limit). Better to have some ABI
> > > consistency between architectures. One can still ask for addresses above
> > > this default limit via mmap().
> >
> > I think that is best as well.
> >
> > Can we please just do what x86 and arm64 does?
>
> I responded to Arnd in the other thread, but I am still not convinced
> that the solution that x86 and arm64 have selected is the best solution.
> The solution of defaulting to 47 bits does allow applications the
> ability to get addresses that are below 47 bits. However, due to
> differences across architectures it doesn't seem possible to have all
> architectures default to the same value. Additionally, this flag will be
> able to help users avoid potential bugs where a hint address is passed
> that causes upper bits of a VA to be used.
The reason we added this limit on arm64 is that we noticed programs
using the top 8 bits of a 64-bit pointer for additional information.
IIRC, it wasn't even openJDK but some JavaScript JIT. We could have
taught those programs of a new flag but since we couldn't tell how many
are out there, it was the safest to default to a smaller limit and opt
in to the higher one. Such opt-in is via mmap() but if you prefer a
prctl() flag, that's fine by me as well (though I think this should be
opt-in to higher addresses rather than opt-out of the higher addresses).
--
Catalin
next prev parent reply other threads:[~2024-09-11 18:21 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-09-05 21:15 [PATCH RFC v3 0/2] mm: Introduce ADDR_LIMIT_47BIT personality flag Charlie Jenkins
2024-09-05 21:15 ` [PATCH RFC v3 1/2] mm: Add personality flag to limit address to 47 bits Charlie Jenkins
2024-09-06 6:59 ` Michael Ellerman
2024-09-09 19:07 ` Charlie Jenkins
2024-09-10 9:20 ` Christophe Leroy
2024-09-10 12:43 ` Geert Uytterhoeven
2024-09-11 13:38 ` Michael Ellerman
2024-09-12 6:20 ` Charlie Jenkins
2024-09-20 5:10 ` Michael Ellerman
2024-09-11 13:37 ` Michael Ellerman
2024-09-06 7:17 ` Arnd Bergmann
2024-09-06 8:02 ` Lorenzo Stoakes
2024-09-06 8:14 ` Lorenzo Stoakes
2024-09-06 9:14 ` Arnd Bergmann
2024-09-06 9:52 ` Lorenzo Stoakes
2024-09-09 23:22 ` Charlie Jenkins
2024-09-10 9:13 ` Arnd Bergmann
2024-09-10 23:29 ` Charlie Jenkins
2024-09-11 13:50 ` Michael Ellerman
2024-09-06 9:14 ` Guo Ren
2024-09-06 9:55 ` Arnd Bergmann
2024-09-06 11:43 ` Catalin Marinas
2024-09-10 19:08 ` Liam R. Howlett
2024-09-11 0:45 ` Charlie Jenkins
2024-09-11 7:25 ` Arnd Bergmann
2024-09-12 6:06 ` Charlie Jenkins
2024-09-11 18:21 ` Catalin Marinas [this message]
2024-09-12 6:18 ` Charlie Jenkins
2024-09-12 10:53 ` Catalin Marinas
2024-09-12 21:15 ` Charlie Jenkins
2024-09-13 10:08 ` Catalin Marinas
2024-09-13 10:21 ` Catalin Marinas
2024-09-13 20:15 ` Charlie Jenkins
2024-09-13 7:41 ` Lorenzo Stoakes
2024-09-13 21:04 ` Charlie Jenkins
2024-10-02 14:26 ` Palmer Dabbelt
2024-09-05 21:15 ` [PATCH RFC v3 2/2] selftests/mm: Create ADDR_LIMIT_47BIT test Charlie Jenkins
2024-09-06 6:08 ` [PATCH RFC v3 0/2] mm: Introduce ADDR_LIMIT_47BIT personality flag Guo Ren
2024-09-06 6:19 ` John Paul Adrian Glaubitz
2024-09-08 11:26 ` Jiaxun Yang
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